my gf got her car serviced (with the NCT in mind) before an NCT and it still failed, she went back to the garage and got all the things fixed FOC and passed on the next attempt, next time she plans to do the test and fix what it fails on, that said its still a good ideal to get your car serviced at regular intervals, check tire threads etc, last thing you want to do is compromise your safety to save a few quid
A mixed strategy is best I think. You should get a car serviced every 6-12 mths anyway, so it's best to try and time these with the NCT. However only a standard service & checks. If they come back with a list of things to be fixed or replaced, I would then wait until the NCT to see in fact if they were needed.