This sort of thing has been talked to death. Why on earth wouldn't you talk to them? It doesn't cost you anything and may save you everything.
Because you're receiving a traffic service from someone else, who will either ask you to contact the MATZ controller or notify them by your behalf or have other knowledge (traffic for example). I'd suggest that it is bad airmanship to dump a traffic Service to then free call a MATZ just to let them know you're there.
I'm still trying to think where this is, where would one be receiving a traffic service from a civillian unit and not the military who's MATZ you're going to fly over? It may be that the civil unit has an agreement with the MATZ, for example Yeovil and Yeovilton (although Yeovil is non radar). One can be in the hold over Yeovil, talking to Yeovil and not Yeovilton, or vice versa because they coordinate with each other. Yeovilton know you're talking to Yeovil due to your sqwark and if they need to call you they will do so via Yeovil.