Well, as an unscientific unsafe test, just put your hand behind the computer. If the air blowing out is relatively hot, then it's probably not safe to reduce the airflow.
Also bear in mind that the fan on the PSU does affect airflow in the whole computer. I.e., slowing it down might also drive your CPU, HDD and GPU speeds up a little.
Personally I would rather get a decent 120mm fan that's silent to start with, rather than slow down a noisy fan to the point where it hardly pushes any air. (Yes, you can see it coming, another plug for Papst) E.g., on my PSU I put one of those 18 dBA fans from Papst, and not temperature controlled either.
It's not completely silent in a completely silent room, but it's close enough.