After years of ambiguity, there is now a legal framework for access to the Scottish countryside which resolves most of the uncertainties. The Scottish Outdoor Access Code provides an excellent summary.
In practice, I've rarely encountered problems with access. There used to be occasional signs discouraging cycling access at places like Little Glenshee and Glen Lethnot, and some discouraging any access at all (e.g. past Redmyre Loch, which was even a public right of way!) but I trust that these have been removed since the new legislation. If you encounter any signs discouraging access to open countryside, you may want to report it to the Access Officer at the relevant local council. The main situations where access is legally excluded are those where someone's privacy is seriously affected, or where temporary restrictions are in place for in-progress management work (e.g. tree felling).
There are, no doubt, the odd landowners who would still like to find ways to generally discourage access, but in the unlikely event that you are confronted with problems, if you know your rights and act reasonably and responsibily in line with the access code, you can't legally be prevented. But most landowners and the people who work on the land have no problem with sharing the landscape with walkers and cyclists, and are often happy to say hello or have a chat. As the code mentions, you need to be aware that other actvities such as deer stalking will be taking place in the countryside at certain times of year, and you can't impinge on the rights of others while exercising your own, so there might be the odd occasion where your planned route needs to be amended.
The access code doesn't address provision of parking. The way you park your car is probably as big a deal, in terms of inconveniencing others and running into disputes, as anything you do once you are cycling. In many cases, the way reluctant landowners discourage access is by ensuring there are no suitable parking spaces nearby, which can bring issues to a head for people who are determined to find somewhere to leave their car, but for a mountain biker this should much less likely to be an issue, as travelling an extra mile or two mile from the nearest suitable spot isn't a big deal.
Aside from your actual rights, there is still plenty to consider with respect to the practicalities, and how you conduct yourself in the countryside.
In acting responsibly, you'll avoid leaving physical marks on the environment as much as possible, and preserve the landscape that you're there to enjoy. Leaving tyre tracks on a landrover track is not a problem of course, but churning up peat or grass on a moorland ridge, or turning a discreet walkers' path into a muddy bog is not going to endear you (or cyclists in general) to anybody. You should get off and walk for a bit if you can't avoid causing damage.
Avoiding disturbance to wildlife and livestock is also very important. It can be hard to know when you are at risk of disturbing wildlife (unless you've just caused a stampede of red deer of course), but the best rule of thumb is not to try to approach it, and not to stay near it for too long. Having your picnic under a tree with a large nest in it, or where a bird had just flown up from the heather, is probably going to disturb nesting, as is standing near an osprey nest hoping for a close up view. See safety for comments on avoiding disturbance to livestock.
You need to use common sense when it comes to sharing tracks and paths with others. Travelling at high speed past other people is obviously not a good idea. Incorporating busy family areas such as country parks into your routes at weekends is worth avoiding.
Hopefully avoiding creating litter is a no-brainer these days, as is leaving gates as you found them to avoid letting livestock escape.
Access & Conduct
MOUNTAIN BIKING in East Scotland
All photographs on this site copyright Chris Stamp, except where otherwise indicated