When you’re moving the harp on the cart and come to stairs, keeping the wheel axle parallel to the edge of the stair will help to keep the weight of the harp from shifting. Same principle applies to slopes and ramps.
Also, watch out for people helping lift the base of the harp, while it’s on the cart, and pulling the harp off the cart by accident. This happens especially when you are trying to roll the harp/cart up the stairs, over ledges, or up onto curbs.
Oh, I just remembered; my DH wrote this:
http://www.harptech.com/Articles/Hauling/Hauling.html
You’ll be fine. Reward yourself with chocolate afterwards.