No rules at all, but personally, I let it have a service with a view to any problems that might arise MOT wise. That way they get sorted before the MOT.
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I think I see what you are saying. If the brakes are inspected on the mot why pay to have them inspected again when it is serviced? A moot point but a service will not always highlight any problems but an mot will. I think you will find your garage will do the mot first and if it fails then correct any problems when the service is done. eg something like pads close to limit will be noted on the mot, even if not a failure item. Also if failed it makes the retest easier.