Repot or not?
 

I have number of orchids of various species with roots coming through the bottom of the pot. Some are just edging out, others have 3-4cm running along the outside of the base of the pot.

I know orchids like being pot bound a bit, so my question is how many roots running outside the pot is enough to tell me that the plant needs a repot?

What kind of orchids? It may make a difference. If they're terrestrials like paphs, then definitely repot. But if they're Vandas, this is normal and you may not want to repot.

None of my paphs are growing through.
The ones growing through are Laelia, Rossioglossum, Zygo, Psychopsis, Polystachya.

Paph's like to be re-potted regularly. I often see spikes shortly after I do a re-pot when it's been in the same one for a while.

With these, I think you should repot only if either the medium has gone bad or the base of the leaves/pseudobulbs have started really growing over the edge of the pot. Some root growth outside the pot is normal and healthy.