It's normally a question of balancing what you want to cool most.
If you have the PSU drawing warm air from inside the case and exhausting it, you are making the PSU fan work harder and under hotter conditions, but improving exhaust airflow from your case.
Personally I find that keeping graphics cards cool and quiet is the hardest task in a PC case. If the PSU can significantly contribute to the case cooling for your graphics cards, it's often worth the disadvantages of having it suck warm air from inside the case. If you already have a pair of 200mm fans in your case, I doubt the PSU can help much, in which case it's best to isolate it.
the debate is over getting cards with reference coolers or with EVGA's ACX coolers that dump heat inside the case.
Exhausting reference coolers for SLI/Crossfire, unless your case is vast/has hurricane-like airflow.
I've done many a dual-GPU rig and you don't want the hot air from one card going straight into another card - get it out the back ASAP.