I will shed from Minoxidil if I stop using it for some period of time. For me the shedding means that it is working and the dormant hairs are being lost and they will move in to the growth phase. The shedding usually lasts about 2 weeks. For me it is only real noticable if I comb my hair. I will see much more hair than normal. I don't wake up with a pillow full of hair or anything.
I have never used propecia, but I would imagine the shedding is probably a good sign.