With one of the earlier downgraders (I believe the 2.5/2.60 one), many people suggested updating to 2.0 and downgrading back again to give a 'clean install' of firmware 1.5. You could also trick the psp into thinking its a 1.0 and then just install the 1.5 update, does the same basic thing.
There was never that many reports of issues if you didn't do this, so I don't know how seriously I'd take it. Guess it didn't hurt to be safe but the newer downgraders don't have the same issue.
If you have a PSP with one of these firmwares I would recommend updating to the official firmware 2.71 and downgrading from there, using one of these guides by evolve:
[Tutorial] How to downgrade your Non-TA082 2.71 PSP to 1.50 [GUIDE] Downgrading a TA-082 2.71 PSP to 3.03 OE-A/B
Make sure you use the right one, depending on if your PSP is a TA-82 version of hardware or not.