The way things are currently, if there is an update to grub2, it doesn't automatically update the bootloader installed. You need to manually re-install it anyway. You just do 2 instead of 1
There really isn't a need to update it periodically, though. Once installed, it pretty much remains the same until you reinstall it (in case of an update, etc...)
But, no, it wouldn't hurt to periodically update them, though.
A lot (most) people don't realize that a normal update to grub2 doesn't update the installed bootloader on their system, and never go back and re-install the bootloader code, so are running with the same bootloader than was put there during the install.