when you have under a 5bb stack 73o and 64o become shoves when in an unopened pot. You can certainly fold them, especially if you have a few more hands before the blinds hit you, but you should be shoving at a minimum 80% with 4 and under BB's. if you find yourself UTG with that stack, then shove 100%, it's better than letting the blinds hit you and being forced to fold BB and SB in an attempt to find a hand "worthy" of calling a shove/raise. if you have fold equity, use it!
if it helps, get pokerstove and punch in an 80% range to find out what hands are included. Basically, you just fold absolute garbage, and shove any high card, any connectors and any suited cards. simple.
so at least 50% for 5 bb's, and at least 80% for under 5 bb's. two simple ranges you can memorize and use in game to make you more profitable.
you have to shove this wide in the $2.50 games, they are filled with a lot of aggro regs, so you don't get many chance to open shove. When you do, take the initiative and the fold equity while you can.