Personally, I'd recommend a minimum of O72, at least on any mainlines. If you do O72, 3.5" clearance from the track centerline, and at least 5.5" separation of concentric curves, you'll eliminate most of the contention issues.
Well kinda. I left 3.5" of clearance from the center rail off the outside 072 curves on my layout. All my largest scale articulated locomotives (SP AC9, SP AC6, DMIR M3 Yellowstone) clear everything fine. I tried running a friends MTH Premier Big Boy on my layout and the smokebox front ran right into a wooden support inside a tunnel. So it turns out 3.5" is fine most of the time, but not always .