"Does it really matter" whether you turn the water on beforehand? Depends on your perspective. In the greater scheme of the universe? No. In your own life? Somewhat. In terms of your comfort? Yes. In the specific terms of avoiding cold water in order to ensure that comfort? Quintessentially.

In other words, there's no satisfactorily objective answer to this question. My real, reflex answer? Fucking yes it matters, how dare you disrespect the holy showering ritual enshrined in collective humanity's very spirit and soul. You will never be clean after such sacrilege.