One rule of thumb that was quoted in another thread (by ducati, I believe...can't remember which thread it's in or I'd put a link herE) is to widen your pattern the further away you are from the source...from close to far, X-Y, ORTF, DIN and NOS. Of course, there's the question of how far is far, and what kind of mics/caps you're running, if you're indoor/outdoor, etc., etc.
--Dave