My understanding is that ServiceCare goes by period, not by number of services?
If so then it is best to purchase it just before the next service is due (I think it has to be at least 30 days prior, or such like), or better still before the next B service, assuming the offer is avalible to older cars.
This will also help to 'capture' more of the 'additional items'.
Personally I did not go for it at the time because I was disillusioned with MB dealer service, but this is another story.