i haven't had any problems with toppling rocks...as long as you "3-point" placement when aquascaping you shouldn't have any problems...
however there is a product that you can use to "glue" your rock together, but i don't remember the name of it (i'm sure someone will know the name)...just remember if you do decide to go that route, don't glue all the rocks together as one big piece or you'll have problems if you ever have to move...