There are pro's and cons for both styles of approach.
When I approach from the drivers side, I will stop well behind the door jam, forcing the stopped person to twist their body to hand me their license/registration, etc. Upon approach, you learn to look for brake lights coming on/off, reverse light coming on/off, as well as making eye contact with the person through the rearview mirror(s).
When I approach from the passenger side, I look for the same things, except usually the driver is trying to locate me, as like most folks, the are waiting on me to appear at their window. This is a benefit as I have seen many drivers/passengers pushing open beers, joints, bags of pot, etc under the seat or arm rest all the while trying to figure out where I am...
Like I said, benefits and disadvantages can be attached to each style. We have no set policy of which side to make an approach and all the academies and training I have had tought both styles. All in all the passenger side approach is probly the best.....