On ZSNES, that doesn't happen (but that's been said a million times). On SNES9x it will not do that if you use the 'stretch (not fill ) screen" option. I rarely use ZSNES, cuz the sound ends up sucking and I have no idea how to fix it.
ZSNES also allows you to perform a neat trick with the SD3 ROM that isn't possible in SNES9x: If you assign a key on your keyboard to "reset" and hold down your keys for L and R and press reset, you'll be presented with a list of songs which you can then play
Umm, how do you assign a key to 'reset'?