Overall, 16bit and 32bit colour depth will be supported just fine. 24bit is pretty much gone these days as far as accelerated support. Old 3DFX users (pre voodoo 4) dont have accelerated support above 16bit. Not sure if thats an issue these days but I guess it depends on your target audience (why loose a potential sale or two as an independant?. Every sale counts).
Of course, 32 bit mode looks a lot better than 16 bit (no banding) on many systems, so forcing 16 bit may just as easily make you lose those sale or two becuase the game is less 'pretty'.
Best practise is to query the display driver and find out for sure whether or not 32 bit is listed as a supported mode.