OK, I just read through 4 pages of discussion on this "quick question"... mostly it reinforces my belief that you can take any subject, and there's an encyclopedia of information about it, and I usually didn't have a clue about any of it!
But I'm still not sure what to do about my small surface plate, purchased on a whim (it was cheap and easily available). It's a 12x18x3 B grade (or worse, totally undocumented), purchased with an eye towards shop work like lapping and/or marking out parts. It's just laying on my nice solid workbench with no support, other than the bench top itself.
If I have to move it, I'll just slide it around like some millstone.
Should I feel bad or ashamed about it?
I've got no welder, and I'm not about to purchase a purpose-made table that costs 20x what the rock did... not sure what else I'd do to support it, but frankly, I'm not too worried about a chunky thing like this sagging.