In general yes task killers are bad. One of the main problems is you end up killing on services that are needed o you waste battery by killing them and they pop right back open in the background.
If you choose to go that way then watch very carefully what your killing.Just a example,but say you see gtalk open and you kill it then next thing you know it's open again,that's cause it's a service that is needed and always runs in the background so you just wasted more battery killing the app and make it open itself back up.
I would say if you don't have root already think about doing that then get you a app called autokiller this will change the lowmemorykiller levels the higher the levels the quicker your phone closes processes it doesn't need.This will give you a lot more ram and usually give way better performance.I personally would never ever use a task killer they are a big waste of battery if you don't know what apps do and don't need to be running at all times. If you choose to use it then i would really do my homework on what is okay to kill off and what is not okay to kill off.