Caching would speed your searches up; however, it would download all the emails you open, and the headers of those you have in your viewed folders.
As far as I know, caching still has a few flaws (like the mailbox list count not updating) but works. It will initially slow down your RC experience; however, overall it will be faster, if you're dealing with only a couple accounts. More than that, and you're risking a overflowing your DB with emails.
So it depends on what you want. Typically, it's just easier to leave caching off. If you have problems with an IMAP connection, turn caching on and you won't see too much of an issue as you'll still read mail, it's just it won't be live, it would be cached.