Chickens are vulnerable at night as they have very poor night vision so they feel safe when they can roost high. They naturally tend to roost in the highest place they can get to. It's instinct for them.
I believe what you may have heard is someone saing that you should put your roosts "higher" than your nests. (rather than above) If your nests are higher than your roosts or even the same height, then your chickens will try to roost in the nests. This can cause issues which I believe have already been mentioned.
I would make sure your roosts are higher than your nests and your chickens will start to roost in the proper place in time. It may take a couple days for your chickens to change their routine, but it should happen fairly quickly.
Best of luck!!!