My main cabinet has a large and quiet fan on the top/back. Probably not necessary in retrospect, but whatever.
I also have a small pedestal cabinet I built, and on that one I decided to forego the fans and just use vents. It has been mostly fine for the 5-6 years I've had it, but there have been several times I've opened it up after it's been running for a while and damn, it gets alarmingly warm in there without fans. That particular cabinet has had 2 PSU failures on 2 different PCs, and I can't help but wonder if the hot environment is expediting these failures. I mean, it's hard to say since the PCs were old and crusty and arguably running on borrowed time to begin with, but it's interesting that I've only ever had 2 PSU failures in my life and both of them are on that cabinet.