Technically, Grub does not require the boot (/boot) partition to be mounted to reference/locate the kernel and boot the machine. Generally, the arguments for having /boot mounted for normal system usage are few. As mentioned, it's been quite some years that 'noauto' has been set as the default for /boot in the template fstab file provided in the stage-3 tarballs from Gentoo and Funtoo. But it's not some type of hard 'law' or anything - if you'd like to always see your boot partition mounted, feel free to go ahead and removed 'noauto' from that line in fstab.
In the Gen/Funtoo-verses (and well, in Linux-verse generally), it's really only necessary to mount /boot when performing a kernel upgrade. Otherwise, it's a best practice to leave it as 'noauto' in fstab, imho. It has happened that newer users in years past have accidentally cd'd into /boot and deleted the kernel. Not the end of the world, by any means, but a pita to correct when you have to reboot the box.