What ever has been said before about camping in Scotland and how it is tolerated and you have free access to go camping, if you do not get the permission of the landowner to camp you are committing an offence under the Trespass Scotland Act of 1865 (yes it is still used), Sec 3 states;

3 Parties lodging in premises or encamping on land, without permission, guilty of an offence.Every person who lodges in any premises, or occupies or encamps on any land, being private property, without the consent and permission of the owner or legal occupier of such premises or land, and every person who encamps or lights a fire on or near any . . . F3 road or enclosed or cultivated land, or in or near any plantation, without the consent and permission of the owner or legal occupier of such road, land, or plantation . . . shall be guilty of an offence punishable as herein-after provided.

Decent campers are asked to move on, drunk idiots with a disposable BBQ burning holes in the grass and leaving litter can be arrested.

So it is best to ask first. Many areas have clamped down such as round Loch Lomond because of the amount of litter and damage done. I would not recommend it in either of the National Parks, though in the Cairngorms, like much of the Highlands you are unlikely to be found if you are sensible.

I have never had any issues camping in Scotland. If possible I have asked and have yet to be turned down. I have been moved on once, but was given a recommendation of somewhere else to go, which we did and had no problems. Once we had a huge spotlight shone on us and then the farmer disappeared. The next day we saw him heading back with his 4x4 with its big spotlight on the roof. He will have found the site totally undisturbed as we even put a broken down gate back in its imprint in the grass when we left.