I'd agree with @malrobb. There are some hills where staying on paths is necessary, because of erosion, and paths are created/maintained to avoid further deterioration. It's most important on popular hills like Schiehallion and Ben Wyvis etc, due to the high footfall.
Wyvis is a good example of this, and people were being asked to keep to the main stretch on the ridge/plateau especially, to avoid more harm being done to the environment. Sadly, some unpleasant characters decided to ignore this and took off road vehicles up there a few years ago, leaving a trail of destruction.
It comes down to a bit of common sense though. If you fancy a wander off the main path, and the conditions, and your fitness levels, are suitable, there's no problem, and you can enjoy a little experimentation.
It goes without saying that deer stalking season always has to be taken into account, as well as being respectful and courteous of farming practice, but I'm sure you already know that.