Sage advice ... right up there with "go ahead and tighten that bolt till just before it snaps" ... <G>
And ya ... some truth to both sides here. Yes, cooler tubes can last longer and do less damage to other internals IF they're still operating within the recommended range. Regular convection can and should provide proper cooling, and a fan SHOULD be overkill if the equipment is up to snuff. Kind of a crapshoot with some of the older stuff, so use your best judgement.
I built a Dynaco based (Latino) kit and didn't much care for how tight the old design was, so I went with a custom case that spread things out a lot more. My temps run significantly lower than the stock setup, even with the big KT120's. Again, not a necessity, but I'm happier with it, and the tubes are doing fine with a LOT of miles on them since the build. Here's the average temps after the amp's been running a while.
PS ... you'll notice a few empty holes in the chassis - early construction there - I just used that pic as it was handy. Also shows the holes I drilled around the tubes so I get good convection cooling from under the chassis plate, which helps a LOT. If your case doesn't have a good sized ventilation screen on top and a way for air to get in from the bottom, I'd go that route instead of a fan.