Ok you guys are all wrong! The D Deck first class entrance was used! I know that from Ken Marshalls paintings and other resources.
Appreciated, Ioannis.You would get the room when you buy the ticket.
The boarding though B or D Deck was optional, it seem it depends more how one arrived at the dock. 1st class passenger Francis Browne (who became later Father Browne) came with the boat train and used the gangway getting aboard on B Deck. Stewards would lead the passengers to where to go and bed stewards would bring them to their cabins.
One more thing... Queenstown... there were a few first class passengers boarding there as well. Would they all board through E Deck regardless of class and the first class passengers then shepherded up to C Deck?