As Big Mike said, a monopod simply doesn't provide much stability when compared to a tripod. A tripod is really required if you want to have images of the highest possible quality. You may not need it in every situation, but there will inevitably be cases where it will prove extremely helpful in producing the sharpest results.
Auto-align might work for monopod HDRs... but I wouldn't recommend relying on it. I suppose it all has alot to do with how steady you can hold a monopod, how fast your shutter speeds will be, as well as the type of lens you're using. The problem you may run into with auto-align is that, if you're using a wide-angle lens for instance, subtle movements of your camera on the monopod may actually change the proportions of certain objects in the scene. If that happens, the individual exposures will never really line up perfectly. On the other hand, such issues will probably be less pronounced if you're using a telephoto lens, since the proportions will remain roughly the same despite subtle camera movements. Either way, producing great HDRs really requires that the shots will line up perfectly. Without a tripod, there's just no surefire way to make sure this happens.
Definitely try to bring along a tripod if at all possible.