Unless you are using test with your deca, then using propecia for the purpose of preventing hairloss is a bad idea. While the 5ar converts test to a harsher, more androgenic compound - DHT, if converts nandrolone to a less harsh, less androgenic compound DHN. So blocking this conversion with the use of propecia means that you will be keeping more, if not all of the nandrolone in its original, more androgenic state not allowing it to convert to the less androgenic byproduct, DHN. This is the reason that nandrolone is perported to be more freindly on hair, skin, etc. So if you take away this function by blocking the 5ar from being able to carry this out, then you'll be making a more androgenic state in your system which is ultimately worse for your hairline.
So stop that propecia right away (unless you are also running test) is your goal is to maintain your hairline.
If hairline is not a concern, then the effects of propecia can be desireable. As more of the nandrolone stays nandrolone with its use, and doesn't downgrade to a less potent steroid, the overall effects is more androgenic - thus more anabolic leading to greater muscle gains.