Like any repair job on a vehicle, there are always short cuts. Changing valve stem seals with head installed is one of them. The proper way to do it, is with the head off. If you want to leave the head on, it can be done if you follow a certain procedure and have a proper compressed air connection to keep the valves closed. For 99.9% of the repair/maintenance jobs I have performed myself on my cars and motorcycles, I have followed an official service manual.