You could do whatever you want and it will still grow with sufficent food. I just tie a tiny sliver of mine on with fishing line to some rubble rock if im selling it , but if your going for keeps just do the same thing in your main tank. Its better if its on a rock because if its on the sand bed, air bubbles sometimes get trapped under them and kill them in spots. Since its a tree sponge better upright growth is encouraged if its attached to something stable.
I usually only frag my sponge by taking a small tip off the top of one of the branches, but I don't mass frag like some people. I only frag it once and wait until the space I have cut off is regrown (just to make sure it healthy and will handle another frag). If you want though you can cut a piece off the tip of each branch.
Good water and feeding also promote the sponges to bud (self frag) more often so look for small sponges in your tank every so often. BTW you will be amazed how much money a single health sponge can bring at a LFS. I frag mine every now and then and raise some of the buds and my LFS buys most of my 5"+ tree sponges for 10-15$! Id say its pretty good considering they ust sit there doing nothing but eat (and make cash lol).
Good Luck