I wouldn't go below 3% aggregate snap reserve (that does let you get back 2% from the default at least) as doing otherwise can cause data corruption issues (as well as cause issues upgrading to 7.3.x).
See this bug report as well....
http://now.netapp.com/NOW/cgi-bin/bol?Type=Detail&Display=164329
There was also a general warning in the 7.3.x upgrade docs about this as well giving 3% as a minimum.
I would stay away from 0% aggregate snap reserve as I have personally had data corruption issues with this (a volume with "file" space guarantee got set to "none" silently and subsequently had some corrupted files which we didn't notice for a couple months due to some odd circumstances).