running water quickly dilutes and carries the urine down the drain, and skipping a toilet flush may save a small amount of water.
The bigger considerations are hygiene, shared-space etiquette, and whether you are developing a habit of urinating in response to the sound or sensation of running water.
So, is it safe? Generally, yes—for most healthy people, in a private, clean shower. Whether it is a smart habit is a slightly different question.
