1. You can do as many as you are allowed; don't make the place you really like your first ophtho rotation; many I've heard do a "home" rotation and an "away".
2. Start as early as possible- I am assuming you are a 3rd year...likely too late for this year (you'd be interviewing during your away)
3. Can help or hurt...it's a definite foot in the door and should be considered an extended interview