Welcome to A/C. Overall task killer can cause your phone to run crappy and sluggish. But on the other end, it can help since some apps does not close all the way when you just hit the escape key.
Best way to use a task killer is to SHUT OFF auto-kill and I highly recommend that you put a lot of stuff on the ignore list. I have Handcent, alarm, browser, fancy widget, tikl and others that are used a lot. Pretty much at the end of the day, task killer is only allowed to kill off games and 3rd party apps I use once in a while.
Another good thing to do is a cache cleaner.