You don't need to uninstall anything when updating it. That includes BootMii/IOS, HBC and BootMii/boot2.
The installer correctly detects a previous version, and substitutes it with the new version when updating, leaving no traces of the old version.
Updating is even recommended, since there's a slight chance that something goes wrong between uninstalling and reinstalling (I'm not aware of any case where that happened though).
When you update HBC, and the title id has changed between the versions, the installer automatically uninstalls the old title id after the new one is successfully installed.
We don't explicitly mention when the code for BootMii/boot2 changes, but you should update it when going from Installer <=0.6 to 0.7 or 0.8. There were no code changes between 0.7 and 0.8. The amount of the boot2 reinstalls don't really mention much either, I for one have installed it more than 100 times (check the "boot2 bootmap generation" in installer.log, this is increased by one on every install).
Basically you don't have to worry about anything, we already took care of all possible cases.