Thats a personal preference. V-sync locks the refresh rate to whatever the verticle max refresh is...in other words if at 1024 by 768 32 bit your monitor can do 85hz...and you have v-sync on..the fatest fps you will get is 85fps.
however with v-sync disabled the graphics card just shoots out the frames as fast as it can, this can lead to some bits of choppiness of sorts during fast action where the fps is very high, you might notice a very slight delay or a slight offset of a texture or such...it's sometimes noticeable in games like quake and all but most people either don't notice, or don't care
from a quality standpoint if you wanted the best image you'd leave v-sync on