We have done hundreds of MB aluminum V8 head jobs (often two a week as this week). We have tapped out about ten. Obviously, we never do it as matter of fact. We do the job and torque the head. If we have a problem we add about 8 hours to the 25 the job pays without tapping and tap all the holes. I have only seen this to be a problem on extremely overheated engines.
BTW the 25 hours is for valve jobs to both heads not for just head gaskets which would be about 17 hours. Because the time is so close it is not likely that we will pull heads off and not do a valve job, though.
Bosch Master, ASE Master, L1
33 years MB technician