I've heard it said before too, but I've never had a problem doing it. I've heard it said a few times the chemicals in the protective padding on the hanger might react with the finish of the guitar neck, etc, but I've never had that happen and I've had guitars (some quite old and valuable) hanging for a long time, some for years, with no problems. I've also heard a few times it was structurally bad for the guitar, which is complete nonsense.
I've never had a problem caused by hanging a guitar on the wall, or keeping one on a stand. Not to say it's impossible (possibly some finish just might react with some padding chemical, which would also be the case for most stands as well), but I've never had it happen. One thing I would like to mention though is that when guitars are hung up or in stands for long periods of time, they are of course exposed to whatever temperature and humidity there is in the room they're hung in. So if the room is exceptionally warm, cold, dry or damp the guitar is subject to that of course. (I keep my house moderately warm and I have a humidifier.) Winter, with warm dry forced-air heat can dry out a guitar too much, so that should always be kept in mind.
These guitars, a Martin and Gibson, have been hanging here
on my office wall for over fifteen years. No problem!
So I'm with Okie on this one. I think the most important thing is for a guitar to be safe of course, but also easily accesible to play (and therefore more likely to practice, etc), and a wall-hanger (or stand) makes that more possible. I use my guitar-cases mainly just when I transport a guitar, or for storage when I know I won't be playing that guitar for awhile.
And welcome to GuitarZone.