Answer: I would cut any dead stems (be sure they are dead) off the plant at any time of the year. Shear or cut all the stems back by 1/3 or 1/2 after the plants have completed flowering every year. This will promote lush new growth and enhance the vigor of the plants. This also prevents the top growth from shading the lower growth which eventually dies from lack of sunlight. If you are leery about cutting the plants back, experiment the first year by only cutting a few plants half way back. You can then tell by observation if this treatment works well for phlox in your area.
Every few years you may have to dig up a few plants or rooted stems here and there to keep the patch from becoming over-crowded. This is a good way to increase the size of your bed or give a few plants away to friends. Where the plants are too thick, a few stems may also be removed by pruning them close to the ground.
Hope this provides some guidance!
Q&A Library Searching Tips