On a set up that shoots 125gr heads, what is the lowest and highest you will allow your heads weigh before you will see a difference in shooting?  I am a reloader so .01 of a grain is often times big to me so I need a little help understanding weight tolerances?

Depends on your set up.  The one I have set up now shoots the same with 145 and 175gr. Heads, so I'd say more then 30.  Tuning is the key.

I shoot 125/135/160 grain heads off all my hunting arrows with no specific difference.

I feel my bow is tuned well.  From lightest to heaviest is 30 grains.  Lightest=120gr Heaviest=150 grains.  I am shooting 125 gr field tips right now.

Same here, I shoot 125's and 160's to the same POI.

I shoot CE Heritage 150's....most of my setups can tolerate +/- 50 gr. before I see a notable difference.
Ken

I can see a change at around 25-30 grains when shooting a group of bareshafts with fletched arrows. If you are just shooting field points I don't think even 50 grains will make much difference using 5" feathers. Broadheads tend to be a different animal though. Because of their surface area at the front of the arrow any extra amount of paradox the arrow has to go through is magnified as the broadhead tries to steer the arrow through the air.

I think it depends more on the archer's abilities than most anything else. Take two shooters: one shoots 4-5" groups at 20 yards; the other groups 10-12". Which archer is most likely to notice a 1-2" shift in their groups?

Depends on your setup and bows. Some of mine shoot well with  45 +/-, others are more sensitive to weight change. You really wont know till you actually shoot the broadheads.......I tune for 160 snuffers and find that the field point weight is not as criticle once tuned......

I see a noticable difference in arrow drop with a 50gr. differnce at 20yrds out of my 45# GM2. My point on distance is 20yrds with a 250gr point and with a 200gr. point its 25yrds. You can really see the arrow drop faster the farther you get out...even if its only 5yrds.