If the studs are in good shape, and the nuts screw on them easily, probably no reason to replace all of them. Probably the biggest problem is overtorqueing the nuts. I couldn't ever find the torque specs in the MB engine manual, but these are small bolts, so I useed about 15 ft-lb.
Anti-sieze is always a good idea on studs going into aluminum.
I just screw them in snug, then oil the nuts and screw them in to 15 lb-ft, no problems.
If you're worried about breaking on off in the head, you could pull them all out, clean and anti-sieze, then re-install just snugly. then if one broke, you copuld probably get it out more easily.

DG