I've seen that when I don't use openings that I still win and even sometimes playing without an opening is better. Until what blitz rating should I not use an opening and just play for fun? I'm currently around 1050 blitz rating.
You have misunderstood that maxim. When you are a new player, you should focus on learning and using general principles of chess- piece development, king safety and such-- rather than trying to memorize opening lines.
That doesn't mean that you should play random moves in the opening.
I've seen that when I don't use openings that I still win and even sometimes playing without an opening is better. Until what blitz rating should I not use an opening and just play for fun? I'm currently around 1050 blitz rating.