I went out on my Shimano Road Wheels that came on my CX bike today, and decided to give them some stick to see what happened. I did 35ml loop using mainly a local old railway route which is currently very muddy, stoney, water-logged etc.. Okay it's not exactly Mtb terrain, but it is close to a lot of CX courses. Anyway, the low spoke count road wheels survived, but there were a few groans from them that gave me the impression that they wouldn't last if continually subjected to the same treatment, so I think 32h is definitely the way to go. I had some built on a previous bike before the cycling boom started to push prices up, and they were spot-on. Mavic Open Pro Rims, Shimano 105 Hubs and DT Swiss spokes. However, I'm sure they would cost me a lot more than the £110 I paid back then. Thanks for the advice