I have quite a few mbuna tanks with extensive rockwork and I don't like to lean rocks against the side glass. I want my rockpiles to be stable and resting just one another. I don't silicone them (never had great luck with silicone sticking rocks together) and I don't expoxy them, mainly because it makes it difficult to break down the tank for cleaning or fry-catching.
In any case, if you have digging fish, make sure you place rocks directly on the tank bottom (or on some cushion that rests on the tank bottom), and then add substrated. The last thing you want is for digging fish (or an overactive gravel vacuum) to undermine your rock structure and cause a landslide.
Good luck,
Jim