Guess who? It's neur0n back again with another custom firmware update! It's 6.60 ME v1.6 -- and the change log says...
•Change CPU clock from 199/99 to 200/100.;
•Update VshMenu. Now you can use translate text.
•Fixed a ISO size when dump it through USB in XMB.
I believe the third item stems from this. So if you've already installed that, and you don't care about translating the VSH Menu or about adjusting the CPU clock, then you need not upgrade ... not right this second anyway. Still though -- it's better than a Sony firmware update ... and it's not mandatory.