If you don't take your propecia religiously and start taking a week off here and there, you might expect some bad things to happen later in life. Hormonal fluctuations are a bad thing, but taking propecia when you're supposed to and making sure to slowly decrease your dose once you stop taking it should prevent any problems.
Scientists still don't know what effect inhibiting 5AR has in individuals later in life. However, it's not like humanity hasn't been messing with hormones for the last few decades. If it's your mom saying this, ask her if she's been on birth control. If she has, then she's actually a very big hypocrite. If not, then she should rest assured that messing with your hormones has no prolonged effect. Just ask the millions of women in this country.