It is all dependent on your needs. Some useful things were added over the years:
- font boldness adjustments (added with firmware 5.8.11 IIRC)
- (official) support for using custom fonts in azw3 of kfx book format (= no custom fonts for old mobi format) that was added sometimes around firmware 5.9.7.
Just keep one thing in mind: before doing firmware updates you should download and install the latest version of jailbreak "hotfix" package from "snapshots" thread:
https://www.mobileread.com/forums/sh...d.php?t=225030
And then after installing firmware update again reinstall the hotfix package.