True enough, but in relation to site built homes, which have no steel frame, or off frame modulars, which also have no steel frame, span tables and the wood used become rather important. My home came standard with 2x8's, but was automatically upped to 2x10's to go off frame. This allowed them to run only 3 lines of support (perimeter and about half a dozen centerline piers), and yes, the floors bounce, not a lot, but enough. I'm going to have to install 2 more rows of supports to eliminate the bounce and inevitable sagging that will occur over time. According to the span tables, I am at the limit of a 2x10 for being able to install ceramic tile (but not natural stone), assuming they used at least #2 grade, it's fine for carpeting though and that's what I have. Had I known this beforehand, I would have insisted on the additional supports.
Properly supported by a steel underframe, you could build a floor out of 2x4's, so the difference becomes how much insulation you can fit. A 2x10 will hold a lot more insulation than a 2x6.
So, then it becomes a trade off.. you can up the size of the joist, with it's increase in cost and loose the extra supports, or go with the small joist and have the extra supports. I guarantee you, unless you demand and pay for it, you won't get 2x6 support structure on a home with 2x8's (or 2x10's).
All that being said.. it is MY opinion that ANY span that exceeds the width of the board in feet is going to bounce (ie 2x6 = 6', 2x8 = 8',etc)