At work we are often in deer encounter conditions. We discussed deer whistles and researched them, finding out they are useless or worse.
In my experience deer whistles make deer less predictable. They don't understand the whistle means there is a car on the road and they should get off the road. Instead they are more easily spooked when you appear. Deer seem to have an instinctive evasive maneuver: when the threat gets close, suddenly wheel around and bound off the other direction. When you see a deer leaving the road, slow way down. It is not unusual for them to dart back the way they came, even if it means they run into the side of your car.
Even though it's clear they are not effective, if you want to know what side to put one on; I'd suggest the front side. Unless you drive backwards more often than going forward.
It really doesn't make any difference because they don't work.
It doesn't matter. They don't work.