Something which has been hinted at but not spelt out is the following:
If you elect to exercise your right to transit a MATZ without talking to the controlling unit you MUST remain clear of any associated ATZ. These you may NOT enter without permission (ANO refers).
On a more positive note, don't be afraid of transiting MATZ. They are relatively simple and, as has been highlighted, if the clearance you are given doesn't suit you you can always ask for one that does. You wil usually get it unless they have conflicting traffic. Better than that, pre-empt it by requesting what you want at the outset.
As has been said already, most MATZ work on the relevant aerodrome QFE, so you can expect to be given a QFE and a height to fly. Some units, such as Benson, will be helpful and work out the difference between altitude and height and give you a height to fly which corresponds roughly to your stated altitude. Be aware also that if you are VFR and are unable to maintain VMC at the cleared height then it is perfectly OK to request a different one.