When to repot (general advice)?

[JungleWriter]

Are there some simple general criteria of when to repot an orchid? My Doritaenopsis has been in the same pot three years, and seems quite happy, so "if it ain't broke, don't fix it," right? But I also bought a few weeks ago a Miltassia and Oncidium Beallara which are both practically crawling out of their pots. The vendor said that they liked to be pot bound so not to bother. But on one of them, the pseudobulbs are wrinkled. So I was wondering if it needed repotting, or what other advice people might have.