I'd say definitely don't go over 180 w/o a custom frame. I ride with a guy who's 6'7". He rides just fine with 175s, did PBP twice. It looks a little odd, but he's fine with it. There have been several threads here and there on BF where folks have advocated strongly for shorter cranks than the formulas say. In general, one can train to be able to put up with most anything and get quite good with it. We get good at what we practice. Without considerable training with each length, it's really impossible to say if one crank length will be of more benefit than another. Spend a year with each length, see what your power is like. Of course that won't work either, because every year is different somehow.
I have a 30" inseam and ride our tandem just fine with 175s. I don't really notice the extra length - my singles have 170s. Zinn's formula says 160. Eh.