I think my opinion is going to be different from most everyone else here, since most follow the direction of Sheldon Brown and Jobst Brandt on this stuff.... The book that I learned wheel building from taught to lace so that the pulling spokes (trailing on a rear wheel [and the leading spokes on a front disc]) should be laced heads in. The purpose of this is to provide the elbow of those highest torque spokes with proper support coming off of the flange. Lacing the opposite will leave the elbow somewhat exposed in it's exit from the flange.

The counter-argument usually has to do with the amount of "deflection" created by the tightening pulling spokes under torque and the possibility that they'll be pulled outward and contact the rear derailleur cage. It's my belief, though, that this does not typically happen, so this argument in most cases is somewhat moot, and completely so when talking about front wheels where there is nothing for deflection to come in contact with. I would rather have well supported spokes where it matters.

I built my last wheel-set with the pulling spokes heads in and have had no issue whatsoever with derailleur clearance. I should also say that if all of the other methods of proper building are used, including stress relief, eliminating spoke twist, proper and balanced tension etc., the style of lacing (heads in/out on leading/trailing spokes) will make extremely little difference in the final product. Certainly not enough to necessitate a rebuild. Hope this helps.

-Jeremy