Probably the best way to approach professors is having studied their research and suggesting some contributions to their work, not too involved, but that at least show your interest and potential.
E-mails that are too generic are usually ignored. Even though the committee makes the decisions, individual professors are usually consulted about their preferences.
I really don't think there's such thing as a "good" time to write to professors, as long as you don't do it too late into the application process.
Sergio