Should you format your Mac? Back in my Windows days, I used to fully format my PC once a year or so, to start fresh and get rid of some of the slowdowns. It helped.

Would you recommend the same for Macs? What's the recommended way of keeping the computer running smoothly?
I'd say a full format is almost never necessary, though the only thing you'll lose is time.

I've got an OS X install that I've migrated from computer A to computer B to computer C and back to B and upgraded from Tiger to Leopard to Snow Leopard (all upgrade-installs, no reformatting or anything) over the past 6 years and it works great.