UV deluxe has a script that does that and it's free http://www.creativecrash.com/maya/downloads/scripts-plugins/texturing/c/uvdeluxe I actually use UV Ninja, but thats costs a few bucks (Worth every penny in my opinion)
I feel like this is such a small question that it doesn't really deserve it's own thread, but I find myself in this situation all the time and it seems like it should have a simple fix. I use those snapping buttons at the top of the image a lot. I realize it's not necessary to have shells precisely on top, but I'm a bit of…