Certified GKH Gardening Expert
It isn't, completely, necessary to cut them back all the way. Usually, you will trim, only the dead flowers heads off. That's not to say that you can't cut them completely. Sometimes, after a few years, they will need a good regenerative pruning.
This article will give you a good run-down of their simple care requirements: https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm