MGZ,
You WILL continue to bald after a transplant. Hair loss is progressive. But propecia will help you keep the hair you have. So don't wait until after a surgery to start. Take it for at least a year before surgery, if not two in your case since you are so young.
And don't psyche yourself into getting side effects. Many doctors state that the vast majority of patients who have side effects were the ones with the most anxiety about taking the drug in the first place. The mind can be very suggestive.