If you can replace your roll easily, then playing with x-amount of buyins doesn't really matter if you're playing for pure entertainment value. If you're playing to pay the bills, then you should probably have about 40-50 buyins for the level you play, so you can A) cash out without having to move down a level and b) enough cushion to make said cash outs every other week. Additionally, playing over-rolled is +EV for a lot of us degenerates.