If you install on top of the previous install it makes life more simple but, as noted by Balthazar2k4, you wind up with a listing of every version installed in Programs & Features which is an annoyance. So, uninstalling the old version first is the cleaner way. That said, if I uninstall the old version and install the new version without rebooting in between all my ODDs vanish. Which means more work to resolve. So, I have to uninstall the old version, reboot, install the new version.
Wish Arusoft could get their update routine to work more like RedFox with AnyDVD where it works very nicely installing the new version right on top of the old.