From the stickied "definitive guide to Wii U hacking"
https://gbatemp.net/threads/the-definitive-guide-to-wii-u-hacking.396828/
"You should only update to the lowest compatibility FW that the Homebrew Launcher currently has. This is so that you can use the hacks supported by the Homebrew Launcher, but you also have a higher chance of being able to use an IOSU exploit if one is found/released.
If you're
under 3.0.0, update to 3.0.0.
If you're
on 3.0.0, stay there.
If you're
on 3.0.1, update to 3.1.0.
If you're
on 3.1.0, stay there.
If you're
on 4.0.0, stay there.
If you're
between 4.0.0 and 4.1.0, update to 4.1.0.
If you're
on 4.1.0, stay there.
If you're
on 5.0.0, stay there.
If you're
on 5.1.0, stay there.
If you're
between 5.1.0 and 5.3.2, update to 5.3.2.
If you're
on 5.3.2, stay there.
If you're
on 5.4.0, stay there.
If you're
on 5.5.0, update to 5.5.1.
If you're
on 5.5.1, stay there."
So if you don't care for an IOSU exploit, or there's a 5.5.1 only homebrew you want to use, then update. Otherwise, stay where you are.