I open the breech when crossing fences, climbing over trees, and similar obstacles. In the case of creeks, if it is just a little stream I can simply step over I wouldn't bother, but a rocky or muddy stream where I might lose my footing or want both hands free then again I'll open the breech. It takes but a moment.
Muzzle discipline, mentioned above, is also part of it, but it can be hard to maintain if you go base over apex.
