Don't upgrade the kernel unless you really need to, for example the driver for your wireless card requires a newer kernel version. As far as other updates go it largely depends on what you want to do and which tools you plan on using. Many of the tools included in BT3 have received numerous upgrades since the BT3 was released, but unless you are going to use a program why update it?
The easiest way to keep your distro up to date is to use an USB installation with persistent changes. This setup means that all changes/upgrades/additions you make during one session will be saved and transfer over to the next time you use BT3.



