BabyBear: I would always call up a MATZ for transit - my question was regarding overfly.
The responses so far reflect exactly what I have been told by other pilots and various instructors also i.e some say no need, some say it is advisable/good airmanship. What I need to do now is to make my own mind up regarding what I will do next time and stick to it, so that I do not dither when the time comes.
It is very useful to hear the rationales (is that a word?) for both sides.
Of course in practice I would go higher than 100ft above if possible in order to avoid inadvertent descent into the MATZ without having requested penetration.