I usually check around 1000 and tighten up a bit. No other checks have needed tightening so far. I check often because my first set of belts got so loose they looked like they were going to skip a tooth. Only takes 10 minutes to check.
Also, it begs the question of how critical tension is. Most people get them put on and never check again until it's time to change them. They're going to loosen up at least a little which means no matter how perfect the tension was it's off after 1000 miles or so until you replace them again. 4V seems more critical but setting 2V belts doesn't need to be rocket science.
Scott
Everyone checks tension cold, because that is where the spec is given.
Have you ever re-checked a belt that is loose cold, when hot?
I still maintain that as long as the belts aren't so loose that they can skip when cold or so tight they make the tensioners scream when hot, it's all good.
Valve clearance is much more important to proper running.