--How important is the receiving line?
It depends on how much you like hugs.
--Do I need RSVP cards if my guests are mostly young?
you can tell them to e-rsvp, and include your e-mail address I guess
--Pros and Cons of opening gifts at the reception?
You might get a nightie or something.
--Who usually cuts the cake (after the first cut) and serves it to guests?
the people that work at the place where the reception is held
--Is it tacky to have them slice their own? Should I assign someone?
there will be someone to handle it at most places, if not, yeah, I'd assign someone.