No single decision you make in your graduate studies is as important as your choice of research supervisor. Not only will this choice affect what you do and who you work with over the next several years, but will also have a profound impact on the direction of your career. The right choice can make your graduate studies an enjoyable and very rewarding experience, but a poor choice can be devastating. Perhaps the most important advice, which almost every graduate student at Toronto agrees with, is that you should not rush into making such an important decision as this one. If you feel you have been pushed to choose a supervisor before you arrive, you have been misled.
Many students do, but there is absolutely no benefit to doing so.
This is not to say that you should not come down to Toronto before September and talk to all the professors you wish. Just remember that most students here believe that you should not rush into a decision before you arrive at school. Try and read the booklet Physics Graduate Studies cover to cover. In this booklet are listed all the professors in the Department, a page about their current research, and a list of publications. At the very least, read the first column of every professor's page where it lists current research.