I think it's really up to the immigration officer (and his/her mood) on this one. I always walk up with my gf when returning to the US. I've never been sent back or told that we needed to come up separately.

Once at SFO upon returning from Asia:

GF and I walk up to booth.
CBP Officer looks at our passports, asks "are you two married?"
Us: No.
CBP Officer (looking at me): Why not, doesn't she ever give you any pressure?
Me: Don't encourage her.
CBP Officer: Ha. Have a good one, welcome home.