As long as it's set up through your phone, you should be alright. The only thing I think I use the BIS website for is resending service books, but you can do that through the phone as well.
You're really not missing much. If you go into your e-mail settings folder, and enter the e-mail settings application on the device, you are going to be able to do everything that you would be able to on the website, albiet in a sligtly different package since it's on the device itself, but it works just the same.
Sending service books is essentially sending the service information to your handset. Service books carry all of your e-mail settings, any push information settings that may need to be on there to use some of your applications, etc. If you ever run into any sort of issue with your e-mail not working correctly, resending the service books is usually the first thing that you should do to see if that clears up the issue. From the e-mail app, once you're at the main screen where you see the e-mail addresses you have setup on your device, press menu, and select service books, then click send service books to re-send them to your device.
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