Every study I've read by animal bioacoustics and behavior experts have come to the conclusion that those ram-air deer whistles don't do anything and, even if the deer could hear them, they would just add more to the deer's confusion/erratic behavior than impel them to run away from the road and your car.
I would say anybody that "swears by them" probably saw a deer on the side of the road that moved away from the road as they approached and they attribute that to the deer whistle when the deer would have done the same thing sans deer whistle. Deer are like women, you never know what the hell they're going to do next.
Also consider the fact that the internals of probably 99% of deer whistles are clogged up with dead bugs. They do lend a certain air of white-trashiness to vehicles, though, especially larger old sedans when the double-sided tape they're attached with gets all soft and they turn at a 45-degree angle one way or another or one's completely gone and the other's still there, the "chrome" finish worn down to gray plastic.
The mention of deer whistles on schoolbuses led me to the question, "Okay, deer whistles but no seat belts?" It seems both sides on the school bus seat belt debate have valid points, so I'm undecided on that argument, but some research led me to this quote, which best sums up the state of some of our schools today:
"There is concern that seat belts could be used as weapons to strike or choke other passengers."
On a side note (or a more relevant note, I should say) I stated earlier that I didn't think the 3M reflective tape would hold up on a skidplate. Unless you skip across rocks and trees on your skidplate, it won't go anywhere. Once that stuff's on, it's on.
I've had to reposition some of mine when adding new stickers, etc. I found the best way to get it off is to slowly slice under it with one of those single-edge razor blades with a thick metal guard on the other side, you know, the kind nobody actually uses to shave with anymore.
If you put a heat gun on the tape like you do some decals and stickers, it will just make things worse by creating a gummy mess.