I'm on a 10mg prescription for "as and when needed" and haven't noticed any problems with exercise at the times I have taken it. All it really does is slow the heart, but it's not known to be a particular issue with exercise I think. It's beneficial in some sports, though that's stuff like archery as it steadies the hand, hence why it's a banned drug in sports.

I have (only just recently) more of a problem not taking the stuff and exercising. Basically after exercise I get heavy tiredness, aches and general anxiety symptoms. This in turn can cause pounding heart. So propranalol helps a bit with that.