Answer: Deadheading is cutting off the blooms, after they are spent and on their way out. It is useful on some flowers to keep them looking tidy and blooming well.
I don't think that is necessary or practical for petunias. They will keep on blooming fine and look great without deadheading. Sometimes an occasional shearing back of the entire plant a bit may be helpful to encourage a flush of new growth and blooms, as well as a bushier plant.
Q&A Library Searching Tips