You should get cranks that fit. The terrain really should not be a big factor in this; you generally want to spin more than slog, especially on a tandem, where standing is less automatic. I have recently gone from 175 to 170 for a much nicer experience (having ridden 170's for 30 years). I don't know why the market seems to be going for longer cranks, but it is actually hard to get 170's in the captain's position these days.