There's really no good way of turning email off and on. If you're on BIS you'd have to remember to log into your carrier's BIS site to turn email delivery off and on each time and that's an incredible hassle IMO.
On BES I think you can disable mail delivery via options on the BB itself. Still, I don't see the point. However, if unread email being delivered to your device bothers you then I'd also say that the BB isn't for you. If you want to specify when you receive mail then you should really look into a pull device.