Just reduce pressure. Think of it this way - a 6" pad has a surface area of 28.25 sq. inches. Let's say the orbital weighs 10 pounds, and you add 5 more when you press. 15 pounds/25.25 sq.in. = about 0.5lb per square inch of pressure.
If you hold the orbital at a 45 degree angle over, say, the trunk/fender bend, most of the pad isn't in contact. The radius of this corner is going to depend on the car, but let's assume a 1" wide strip of the pad is in contact - i.e., 6 square inches. With the same pressure, 15lbs/6 sq.in. = 2.5 PSI - an increase of 5X. SO - you should actually support the most of the weight of the orbital and end up with a more comparable pressure to that when the whole pad is in contact.
the shorter answer is - use reasonable judgement. The problem with edges and corners is with rotary buffers. It is VERY difficult to burn trrough the paint with the orbital. Nevertheless you should be careful since clearcoat might be thinner there.
