You know, if you just spread the ASICs out (I'm assuming you're talking about single-chip usb sticks, like red furies or U2s) you won't even need a fan.
Give them maybe 2cm of space from each other and they'll be fine. I ran ASICMINER block erupters like that for months, and got no problems.
Multi-chip miners I've heard definitely need fans though.
It'll save you a bit of cash!