I dont think it should be much of a problem as long as the bulk of the rock isnt leaning completely on the glass.
Some people just throw their rock in and let it form a natural aquascape by itself, but those looking for more control can arrange by hand. Often you can make a nice structure by keeping the rocks in certain ways depending on their shape. My rock is not typical marine rock but you can look at it for an example of how to fit them together (pic
). As long as you dont have any fish that would be burying or pushing against the structure it will be ok. You can also drill holes in the rock and use acrylic rods to hold them together, or simply use zip ties. A last solution is to silicone them together, but this is a bit more permanent.
Marco Rocks has a page on aquascaping, click here
Hope that helps