It depends I do it as soon as someone touches my car or tries to talk to me in person like if he walks up to the car asks me to roll down the window to wait a few I start it. The moment I interact with you in person that is a paid interaction imo.
Depends of your parking situation, if you on a no stopping road whenever the customer is walking towards the car start the ride this way you gonna get less people showing you the finger for stopping in a non stopping area
Safest is when the rider is in your car and you read them that they won't be trouble. This way you can refuse to ride them without getting a hit on your rating. Also you'll be much safer since you won't be driving some a.hole.