I get this question all the time. How important is it that a welding table be flat? Also what’s the difference between a welding table and a workbench? I’d like to share my answers to these questions but I’d like to hear what everyone else’s opinion on this is.
A workbench is just that, a work bench. Plate steel, 1"-2" in thickness, flat to ASTM standards for plate of that size. Four legs, not mobile, stout enough to really hammer on and not move the table, shelf underneath to store tools often used. The workbench might have an automatic transmission torn down and spread out across it for rebuild. A place to tear apart an item(s), perform a task, then reassemble with all the parts in once organized place. At the same time, can be used to fabricate on. Building a framework out of square tube, setup a drilling operation with the mag drill, using the edge to hammer a short cone gore into final shape. In the end it’s a sacrificial surface to get any sort of work done.
A welding table would be a table setup for just that, welding. It has a different set of tools to perform a more specific task. How flat should it be? That depends upon the industry of use. Does every fabrication shop need a welding table to be within .002" flatness? Most fabrication shops I visit don’t know what thousandths of an inch even means. I have personally worked inside of 6 different fabrication shops within the past month. All varying in work quality and facility quality. Some of the shops are job type shops, others are production oriented. The one thing they all have in common is the brand of welding machines and plate steel welding tables used to fabricate on.
I would think any one doing fab work would know what .002" means. But I have heard many guys read a tape measure by saying it 36 - 3/4" plus a tick more. 
I would not call them Fabricators.
If you worked in 6 shops in a month you must be there doing other work correct? Or you hold a record for most places worked in the shortest amount of time. 
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My grain of salt practical opinion (home/hobbyist edition):
Workbench: Metal or wood table you do everything on. From welding to fabrication to gunsmithing to transplanting your pineapple plant to a bigger pot. You use and abuse the workbench. My workbench is on wheels because I have one stall of a 1950’s two stall garage for everything. Flatness shouldn’t be worse than 1/8" corner to corner.
Welding table is a metal top table (with or without fabrication holes) dedicated to metal fabrication and welding. (Two subsets: general welding table and a fabrication table).
Flatness should be 1/32" or flatter end to end. The difference between the two subsets: holes, and if the owner of the table will cut your fingers off for welding a piece to the table to hold the piece.
My workbench is converted from a trebuchet I made in college. The 3/8" table top plate is a scrap piece I bought from my old job. I beat on it, do gunsmithing on it, do gardening on it. That trebuchet was wayyyyy over-engineered. She's sturdy. She's husky. She's cornfed. She'll go fishing with you on a moment's notice.
My welding fabrication table is a premade bought one. It needs to be flat. It uses your fixture tools. I will never weld a piece to it, and I will not set crap on it for storage when I'm not using it.....except my radio. Setting a drink on it would be akin to burning a holy book.
It's a primeweld table. When I'm in a better place financially, I'm getting a Fireball dragon table. It was either get the cheaper table and be able to have fireball tools, or get a dragon table and have no fixture tools.
Ohhh, thanks for asking! You get a pineapple from the store. Cut off the stem about an inch from the leaves. Annnnnd, plant that in a large pot. That’s it. Mine has survived 6 years. Once Fall comes, It comes inside for the winter (I live in Iowa). I think it’s cool.
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A welding table needs to be flat enough but not thousands of an inch over six feet. How it is un-flat would be more of an issue. A hump in the middle that causes a flat peice to rock is harder to deal with than one that dips across that span.
A welding table is a metal structure with holes for fixtures. My quick response to a workbench would be something without holes and usually made from wood. Then I thought about many of the woodworker’s benches and they usually have holes like the Roubo style I made or the English Nicholson bench. A workbench needs to be stout enough to be hammered on without damage or moving around.
as flat as it can be. it works as a reference table too. and it allways needs to be leveled perfectly 
How flat is a piece of string?
There’s a word, the understanding of which i find differentiates people. It is “context”. Every question exists in a context. “What is a welding table?”; “How flat does a fabrication aid need to be?” As any, these are questions per context.
So “How flat should a welding table be?” - As flat as it needs to be (to meet the requirements of the context).
. Now people might not like to hear that, because of the consequences for accepting this truth. Which perhaps aren’t what one would expect. For instance, choosing the particular horizontal platten fabrication aid to actually physically have, requires something of a person. That they know this context. That they know their requirements. Including thinking, planning, predicting, what they might need in future. Because the investment (time; interruptions; etc) of swapping out tables is probably something one wants to avoid (even more than avoiding over-spec’ing). So an honesty with oneself about all sorts of things. Like what’s achievable, and what’s one’s personal plan / goals. So the context isn’t only the next job or what you know you’re going to be building.
. The question might even exist outside of the characteristic of flatness. How flat does the welding table i choose have to be? - As flat as the one is which i buy because it works with the type of tooling that i need. For example.
. So the answer is as broad as the context. And the context is as broad as the range of cases out there in the world, including future contexts.
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Flatter than the butt of a 1980’s 45 year old arobics instructor…
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