Sorry bud, but if you have to ask this questions you shouldn't be on any beta program, beta doesn't mean early access to show off to friends.
On the other hand, yes, betas have an expiration date, no, you probably won't be able to stay on DP5 until RC, and yes, you should always update to the latest beta.
That said, if I were you, I would rather install the Public Beta, PB is almost identical as DP4 which is pretty stable, yes, far less updates but that's because Apple is releasing with caution since is not for developers and more for a enthusiast of new software that wants to try it and help with feedback, and that's you, since I don't believe you are a developer.
Don't want to sound rude, hope it helps.