not sure about rogaine as yet..but if you're not losing hair and you take propecia then it'll just act as a prostrate drug and nothing else. maybe you'll face sides due to DHT reduction but thats about it. No shedding if thats what you're worried about as DHT wont be the culprit and reducing it wont cause a shift from telogen to anagen.
as for rogaine, i've heard different stuff..some said it'll boost the growth somewhat, while others said since it works differently the propecia that it 'might' give you a shed. again..not sure here, maybe somebody else here can shed light on this subject.