I don't - I use ctrl-s instead. Similar position so just as easy to hit, but it seems to clash with fewer important terminal keystrokes as it has been historically used for flow control in the terminal driver. Here's the relevant bit for .tmuxrc:

  unbind C-b
  set -g prefix C-s
  bind s send-prefix
  bind C-s if '! tmux last-pane' last-window