My tank is a standard 55 gallon so I don't have much room between the front and back of the glass so my rock work is leaning against the back glass.
I also allowed it to go up against the sides, which as mentioned makes it impossible to clean, so the sides are algae covered, as well as the back.
And also as mentioned it does make apatasia and such that much harder to deal with.
I actually have not had a problem with scratching though.
Oh and another tip! don't, however good it sounds, DON'T ever try to hide your power heads by burying them in the rock work.
I didn't have a problem for two years, but now the screens are getting plugged up and it's trying to make them run hotter which heats my tank. Now I have to tear apart half my wall to take them back out.