Saying no to a teenager staying alone, overnight, in a house that you own and therefore where you would have liability....not a parenting decision in my opinion, but a homeowner's decision.
Deciding that the exchange will take place when mom gets off work, even if that makes it 2:00AM....100% parenting decision and therefore not the business of the stepparent to express an opinion unless asked.
Its a pretty simple situation. If something directly effects a stepparent, then I don't have a problem with the stepparent expressing an opinion. However, when something does not directly effect the stepparent, or the stepparent does not have to be effected if the stepparent chooses not to be, then its completely a parenting decision, and the stepparent should not be expressing an opinion unless asked.