Depends what PSP you have. I don't know a great deal about firmware's higher than 5.00 M33-6 (which is what I have on my PHAT 1003).

However, I do know that you can't have any real permanent CFW on a PSP 3000 or GO. If you have a 1000 or 2000 series, then I'm sure there are permanent 6.60 CFW's.

If you're on a 3000 or GO, you could just use standby all the time (unless you want to keep enabling CFW).

Additionally, is 6.60 necessary? From what I understand, you only need to upgrade firmwares as and when to be able to play latest PSP games without patching.