There was a good thread on this a short while back:
George Dixon (Hunter54) had one of the better posts on bare shaft tuning.
Hope this helps,
Bare shaft tuning requires a great deal of skill.
If your form is very solid,
and your bow is tuned well,
then bare shaft tuning will help you find the best combination
of draw weight, and arrow shaft length and arrow tip weight
to get the tightest groups.
Since a bare shaft also has zero steering correction,
then shooting a bareshaft will help you find the best position
for the nocking point/d-loop so you get level bareshaft flight.
You can even try creep tuning with a bareshaft
so you can really fine tune your cam timing.
Shooting a bareshaft for groups,
however, will also shine the spotlight
on form errors.
It's a powerful technique,
one of several tuning techniques.