I'm sure Calvin can give you more insight but if you're checking for mail at such short intervals and have other "crap" leeching data off of your connection, unless every single one of those apps is synchronized to the same intervals and pulls the data at the same time, don't bother.

I mean, you'll have to activate the "Always connect" thing on Settings, so anything that needs network access will bring the link up when it needs it. You could set autodisconnect to work at 1 minute intervals and put a relatively high threshold there, in which case it'll bring down the connection at the latest 1'59'' after the last big chunk of data was downloaded, but if the connection is going up and down several times within that 5 minute interval (of your mail widget/app) then it's probably not worth it -- the added overhead will likely negate any savings on battery.