It Would Depend...

...On weather the grade is towards away from the throat of the yard. If towards the throat you would always have to leave a loco on each track or at least some form of "brake" to stop rolling stock running away, If the grade is away from the throat i don't see a problem as long as your locos have sufficient power to start the trains on the 1.2% Grade.

Personally I wouldn't do it as I plan to "fiddle" the cars in my stage trains on my soon to be started layout. If it was on a grade then taking a car out of the middle of the train means the rest moving down grade not really satisfactory in my opinion.

fiddle or store

Thanks Graeme  So some use the staging track to store trains and others "fiddle" meaning rearrange the cars.

So I dont have the the room so i could have a staging tracks hidden under the mountain so as to store but no fiddling, is what you are saying. 

Staging...

...would probably be OK. You would still need to able to reach the tracks at minimum to remove cars if derailed.

My layout is projected to have a 2 track storage yard behind buildings that can be removed if I need to do maintenance but there is enough room for me to reach in and remove cars if derailed. Do not totally bury a staging yard without having access for maintenance because Murphy's Law says that is where you will have a problem.