Personally, I didn't.
I own 1/4 share (with a 50% owner and another 25% owner). I looked over the plane carefully and flew it twice before writing the check. I got to know the 50% owner who was selling me another guy's 25% and I have good reason to trust his judgement on the condition of the plane. I knew he and the other guy fly it very regularly. Additionally, it wasn't anywhere near the cost of a new plane and it was new enough I was comfortable making the purchase with minimal fear of sudden maintenance needs (plus a good maintenance reserve helps too).
However, I don't think it would be unreasonable to perform an inspection, either -- perhaps you pay for an annual at your choice of mechanic, and if you end up buying, the other owners pay you back for their share of the inspection?
It's your money, so you have to do whatever you judge makes you comfortable and safe.