I don't bother with rocks when cooking with wood. When there are some coals, I put my pot directly on them and build sticks up around the outside of the pot.

Pots on rocks tend to be wobbly, sometimes very wobbly, and if the rocks are wet, they have been known to explode, making cooking a bit too exciting. Plus you're left with blackened rocks.

Admittedly, I don't cook on wood any more, but I did for many, many years. I may start doing it again to conserve fuel. However, most of my backpacking is near timberline where no fires are allowed, and in August and September when campfires are generally banned in the Pacific NW, sometimes even in campground fireplaces. Often when fires are banned, wood-burning stoves and sometimes even alcohol stoves are banned, too. This seems to vary by jurisdiction and even by USFS ranger district.