You should get some varied answers to this question as some people on the private forum seem to be MATZ-haters! It is absolutely true that you are under no obligation to contact the ATSU as you are outside the MATZ and the ATZ.
However, if passing through the overhead only 100ft above the MATZ ceiling I would almost always make contact. You are likely to get a good service from them, and both parties will be more aware of each other's movements. I think it's good airmanship. Additionally, if you had an engine failure, whose airspace are you going to be descending into and who do you think you might want to talk to?
Don't be afraid of talking to military air trafic. They are normally extremely helpful and accommodating. Also don't feel 'uneasy' about 'request'. You don't have to request anything if you don't want MATZ penetration - you can merely inform them of your intentions. (You may want to request a service though).