Common trend is that an amateur boxer should be able to jump rope for half an hour, does this include alternating feet?, or only jumping with both feet at the same time because that is harder.
I usually do different foot patterns. One reason is that it makes it less boring. And another reason is that it conditions your lower extremities for all those movements and not just for one. Check out Buddy Lee's jump rope video. He pretty much covers the usually variations you can try like the side/forward straddle, skier's jump, bell jump, etc. Eventually your body becomes efficient at doing those movements. To continue to progress at that point, you can start doing sprints and double unders. http://books.google.com/books?id=tbEKSuaMl_UC
There's also this guy, playing a Rocky soundtrack certainly never hurts. [ame]http://www.youtube.com/watch?v=lVkYjp4mqnc[/ame]
I think that some form of alternating feet should take most of the time. A large part of skipping as training and not just cardio is shifting ballance from foot to foot.