RE: Hanger 9 tach instructions
Mine also seems to fluctuate, but I'm not sure if its because the tach is having a difficult time picking up the prop (APC props are all but invisible when spinning), or if its just the low quality of the tach. Personally, I think its the latter as one of m friends has tached the same prop with his tach and it definitely doesnt fluctuate nearly as much as mine.
Theoretically, the tach can pick up the prop from the front or behind, as long as it sees something spinning. However, as you guessed, safety is the main reason. No sense needlessly being in front of the prop when the tach works just as well from the front. (Excluding any paint or something that makes that front of the prop a bit easier for the tach to "see")