I dont know for sure, but I do know how hand held tachs work.  And they basically count light pulses between the 2 blades (even though a prop is one blade).

So, if you point it to a wide area of the prop and take a reading, compared to a reading that was taken at a more narrow section of the prop...I could see that there would be a difference in the RPM...since the distance between the two wide areas of the prop would be less than the distance between the area of the thinner section.

I would think you would want to take your readings as close to the outter edge as you can to get a good reading.  I would think that if you take it further in...you would not be accurate for the reasons I stated above.  Also...and this is me thinking without "thinking"...seems that the smaller circumference(sp?) near the center would give a false reading than out near the tip.

Hopefully someone that knows better will chime in and we both can learn something.  I dont really tach my props simply because I have a crappy tach that battery is always dead when I need it!  lol  But when I have used it...I've always took the reading near the end of the prop.