I think there is a fine between between keeping the elbow tucked in nicely for protection, and getting it up in the air enough for proper rotation on a cross. I see a lot of crosses that look like baseball throws because of it being too low on the follow through. It's an ugly thing to witness, kinda looks like the flight of a wounded bird. And then on the other hand, is the high elbow punches, which though not quite as ugly to witness, is an invitation to getting your liver smashed in.