I generally text a few hours beforehand. If the time works then it's all good to go. If not I'm always super polite and make sure to cancel with early enough notice and not last minute. Having good texting etiquette and being polite can go a long way. Handlers and girls themselves will appreciate someone who is easy to communicate with.
On another note, I also kind of agree that during a session when a girl texts it's a bit annoying. A few texts to confirm next client is no biggie, but straight up texting during is a session is annoying. Had that happen a few times, where the girl is texting her other pooners to reel in bookings, and they're having straight up conversations. Pretty annoying suffice to say, have not repeated with them. But luckily it's a rare situation that I've run into.