I'm on propranolol daily for SVT from Wolff-Parkinson-White & my pulse drops to 45 or lower at night. Doc said it's ok and just stay on it?

Probably. With that story, i would not expect you to have any symptoms, ; i would not be concerned unless you had symptoms. Of course, at your age, i would typically recommend an electrophysiology study and possible ablation to attempt to cure the condition.
Probably. Not significant if you don't have any other symptoms, such as fainting or dizziness. Consult with a cardiologist to make sure.
Good advice. At your age with wpw, i would also recommend an electrical study of your heart to help evaluate your risk for developing serious heart rhythm problems that could lead to fainting or sudden collapse. http://www.hrsonline.org/practice-guidance/clinical-guidelines-documents/2012-management-of-the-asymptomatic-young-patient-with-a-wolff-parkinson-white#axzz2td6p9v5a.