When placing rocks in the tank do you just stack them in a way that they sit atop each other or are the held in place by some sort of glue like substance?
Yes. I know some people who drill holes through their rocks and run acrylic threaded rod in it to create pilars. Possibilities are endless as long as it's not toxic to the tankmates.
I just stack. I considered drilling holes for pvc, too, but I was always in a position where I need to put my rocks in immediately. I considered shaping my home made garf rocks w/ holes in them to slip pvc in, too.
I would say setting the rocks up on top of each other is the best unless you have planned out some incredibly amazing aquascaping and want it to be like that for a really long time....
If you do just stack, make sure that rocks are very secure. Just imagine a rockslide in a glass tank filled with water . . ..
I personally l like the zip tie method. Just zip tie them together, and then if you decide to change it then you can easily snip the ties and rearrange.