for winter ball do you mean playing in a weekly event, like say the Batter's Box, or do you mean winter tourneys? As my DD has transitioned into the 16u age bracket and school-ball is looming ahead in the Spring, she's been doing a little bit of all 3.
Practice is always encouraged, if a team can find the space. If not, a committed girl should find the time (normally a committed girl already has a committed dad/mom) to work on skills at home, wether it be weight lifting, swinging off the tee or running. I didnt think that every bit mattered, but now that DD is a freshmen, i can see the hard work has paid off compared to the girls that play other sports or that do not take the time to play competive travel ball.
Anyway, a weekly league serves a mean to an end. Pitchers and catchers get the best amount of work, but outfielders get zero and every girl wants to play short anyway. So, you have to make sure you are playing quality competition, which means a varsity league, but if your DD is 12 and impish, playing against girls that probably have nose piercings and live-in boyfriends is not going to help.
Now winter tourneys is something else entirely. Playing in the right tourneys will pay dividends, if used along with some good practice