RouterBOOT (firmware/bootloader) rarely needs updating, and there have been no vulnerabilities in it. Just make sure you upgrade RouterOS, that is most important
How can we tell when it does need upgrading?
I recall a time when there was a separate PoE firmware several years ago. Then, it was integrated into the RouterBOARD firmware. I would keep track of different versions and update as necessary. It would be very useful to have some means to know what the RouterBOARD firmware changes are, so we can know when to update it.