I think what's more important than practicing or not practicing is how you approach playing outside of competition. The more disciplined you are about it, the more you'll improve. How disciplined you should be is really a matter of your personal taste and what you want to get out of pinball.
Personally, I'm very disciplined about always playing in focused way. I try to control the ball at every opportunity, be strategic about what I'm shooting at and refine my strategy as I get better at different shots, and always learn from my mistakes. I die a little inside with every single ball that I lose. It's pretty intense, one time I was concentrating so hard that I squeezed the cabinet until the playfield glass exploded (just kidding). It's not everyone's cup of tea, but I do think playing that way has helped me improve a lot in a short period of time.
What I'm not disciplined about is putting in the time to learn lots of different games. I like to mostly play games that I know pretty well, and learn a new one once in a while. I'd be a better competitive player if I made an effort to learn more games, but I just don't want to, pinball would be less fun for me if I did. The exception is when there's a game that I really hate, and I get burned by it in competition. Then I'll put in some effort to learn it a little bit, and I love it when I can a good result on a game that used to kill me.