Majority of snes (and nes) games are designed at 256x224.
For those with ArcadeVGAs, the best match is 256x240 obviously.
NTSC is generally considered as 720x480, so doubling the res above to keep aspect ratios takes you to 512x480. This is why stretching is required if you are outputing it to a TV.
Not the prettiest solution IMO, but hey, playing with horizontal borders aint pretty either.