Openings easily start to matter at 1400-1500. A '1700' player can usually overcome not knowing the opening lines with better tactical ability against players under 1600 or so, but it's not guaranteed. It's a rather antiquated idea that openings are reserved only for players over a certain level, especially these days given the immense amount of tools and training available to new players.

At the very least, all players need an understanding of opening fundamentals.