I generally wear undershirts at all times. Is that still acceptable? Also, what is the best undershirt (tee, not tank)?
Best Answer So Far
It's acceptable, but it really shouldn't be. At the risk of sounding like an elitist a******, they look a bit low class. For a rule of thumb, if you can tell you're wearing an undershirt underneath, ditch the undershirt.
Undershirts were worn so that you could wear shirts more than once before washing. If you live somewhere where it's stupid hot, and you're going to pit out your shirt, then the undershirt is at least understandable. But, in the age of air-conditioned everything, cheap dry cleaning, and shirts you can throw in the washing machine, there's no need for them.
If you find your shirts are not comfortable enough to wear without one, get some new shirts. You can buy cheap dress shirts that are made out of good fabric, but they usually don't fit well. If you buy them at Nordstrom's they'll tailor them for you for free. If you buy elsewhere, you can have any seamstress take in an inch or two off each side for ~$5, which will give any shirt a more tailored, custom-fit look.