In my experience, Scotland is fine for wild camping, but I only do this in the highlands and as far away from people as possible.
As already said, arrive late and leave early and don't leave any trace of your stay. Bury any crap, pack out all rubbish and don't damage the environment buy cutting down trees, etc. My camping location is usually determined by where I can find a source of clean running water (stream or river, so needs to be found in daylight).
Forestry Commission lands, I think, often ban wild camping (with or without a vehicle) because of the risk of fire, but if you arrive late/leave early, are discreet and use common sense, it is unlikely you will be bothered.