I don't consider 30 days before calving to be too critical a time to haul, but definitely give your heifer plenty of room on the trip. Hauling will cause some stress, and that's the worst part about the situation, so make her as comfortable as possible, and you'll help minimize the amount of stress and any possible negative impact. The more calm or easy going your heifer is, the easier that will be to do. Crowding her or packing her in like a sardine would not be recommended. She should be fine.
Hauling one just before calving is a lot safer in my mind than having to haul a newborn baby, so... The sooner the better.
Good luck!