it doesn't hurt to repair permissions, sometimes something could be going wrong that you know nothing about, and it will correct it. same for defragging, it's not really necessary but can't hurt. i defrag every 6 months or so... force of habit, i guess.
the biggest thing i think is letting the computer be on and awake at night, i let the screen go to sleep but the computer stays on... your computer does nightly, weekly, and monthly maintenance routines if it's awake at like 2AM. There are ways to run these routines otherwise, like MacJanitor, but if you just leave it on at night, it takes care of itself.
pnw