--How important is the receiving line?
I didn't have one.
I had a second line parade instead.

--Do I need RSVP cards if my guests are mostly young?
Only if you want to know how many people are coming.
It forces people to answer you on time.
--Pros and Cons of opening gifts at the reception?
Don't do it. Open a bottle of champagne and do it while tipsy the next day. Much more fun and less awakward for friends who could only afford small gifts from your registry.
--Who usually cuts the cake (after the first cut) and serves it to guests?
The servers did it for us.
You'll want a pro doing it.

--Is it tacky to have them slice their own? Should I assign someone?
If that's the option, assign someone.