Normally, I skip intros and ending themes if I've seen them before or if I absolutely hate them. If I don't, chances are, I like the theme(s). I do check to see the previews and extra stuff in case the episode has it. Who else does (or doesn't do) this and why?
In repeated showings of the opener and closers, I normally skip them. If it is the first time for me to see them, then I normally watch through them to see what they are like.