I use uberTwitter. There is a Twitter for BB as well.
I use twitter for BlackBerry. You can select this to update in the back ground (even if the app is closed ever few minutes) but to save on battery I have it set to update only when opened.
I use Ubertwitter. you can customize how often you want it to update. I have mine set to every 20 minutes. I would think perhaps 10 minutes would be safe. Anything less than that and you might start heavily affecting battery life....
I have better luck with SocialScope set to "Auto" refresh than Twitter for BlackBerry set to refresh every 30 minutes.
I've used UberTwitter a few times, but it's way too much app for my needs (I hate the almost never ending menu). I know a lot of people swear by it though...
I think battery life on Twitter apps is very dependent on what you set the refresh rate to.