When to trim flowering trees?


This is sort of an odd question, but I figure someone here will have the answer. My folks have a dogwood tree that is currently in bloom, but they want it trimmed. I prefer to trim trees before they "wake-up" in the spring. My dad is telling me that you trim flowering trees once the blooms are gone. Obviously my fear is killing or harming the tree or doing something that causes it not to bloom. The tree needs shaped so it will be a healthy trimming it isn't just a branch or two. Do Iwait until next spring, or wait until the blooms are gone?