If you want to kill off an app, without using a task killer or process manager or whatever add on... do the following:
Settings > Applications > Manage Applications
hit menu, hit filter, select Running
the displayed apps are running. Kill off anything you think you dont want. Just remember, do not kill off clock if your using an alarm, don't kill of calendar or processes like com.android.googlesearch or things you are just totally unclear on. If it shows Pandora/LastFM, and you don't want it on, kill it, if it shows albums and dont want pictures on kill it. I only recommend killing off 3rd party apps that you have downloaded, you'll see plenty of wierd .com stuffs that are stock processes (even if you kill them they turn back on so dont bother). The stock task/app manager is a very inefficient way to kill apps though as the stock task killer is VERY SLOW to load stuffs. But it works if your curious to see whats running. This is why many people do opt into downloading 3rd party task/process killer/managers. But thats another story for a different post (which have been posted countless times, and will continue to be posted in the future).
Your phone should be restarted once in awhile, but also the more widgets you have running, the more cool stuff you put on your 7 home screens the slower your phone will run because your running more things at once. So keep in mind what you definitely want to have running always, and what you rarely use. If you dont use it daily, dont use a widget. Just use a short cut. That should cut down on some MIPS usage (CPU cycles).