There was a thread or two long ago when the Eris first got root discussing all the things you could delete from the phone without having a negative impact.
Every time I flash a new ROM I go through and delete things I don't use or want like twitter, amazonmp3, and the like.
Maybe that thread is still somewhere in the All Things Root section.
The cache2cache (by zach.xtr) thing is pretty slick too. I haven't seen that warning in ages and I owe it to cache2cache and that delete button