I've done these at the end of the workout (8# sledge). Really opens up the shoulders, especially after all that benching. Now that you bring it up, though, It might be good to do as a warmup to benching/pressing. I'm currently trying to work up the courage to do them one-handed.
There's also what I've heard called "halos", done with either a plate or kettlebell. These will also warm up the shoulders, and are gentler, because the mace swinging can be a bit more dynamic (you have to be ready for the downswing, cuz' gravity/acceleration).