Every time I go to the optometrist, they want to slightly change my
prescription.  I always resist these changes for the following reasons:

1) I used to go to the optometrist yearly.  Every year, they would change a
lens (and it was always the opposite lens as the year before).  They
wouldn't change the prescription much, but as soon as they would change a
lens, my opposite eye would get a bit lazy and lose some of it's acuity.
Ironically, when I quick going to optometrist, my vision never declined any
further.  Yes, the opposite eye lost some of it's acuity, but as long as
they weren't changing lens, it never declined.  Thus, I am somewhat
suspicious that lens changes permit my eyes to be lazy and they decline.
Whereas, if I make the eyes work, they won't decline.  I should point out
that the corrections they are making are never that large, and my vision is
usually 20/20 (or slightly better) in the eye they did the most recent lens
change in, and something just less than 20/20 in the other.

2) My vision changes from day to day.  If I've had several good nights
sleep, and it is early in the morning my vision is clear.  If I have been
working long hours (with little sleep), my vision will decline throughout
the day.  But my vision always seems to restore itself once I've had a few
days of rest.  Consequently, when the optometrist wants to change my
prescription, I half suspect that on a different day, the results of the
test may be different.

3) It is very costly to change my prescription, because I have contact lens,
and two pairs of glasses.  I find that when they change my prescription
(even when it is a slight change), it is hard to go back to the old glasses.
Consequently, a prescription change means a large cost.

Currently, my lens in one pair of glasses is all scratched up, and they will
not replace lens without a current prescription.  Thus, I need to go back to
the optometrist.  How do I ensure that they aren't "over-prescribing"?  The
last optometrist wanted to change the prescription of my one eye slightly,
but when I challenged her on it, she agreed it wasn't necessary.  But
educated professionals don't like to be challenged.  What is the best way
for me to challenge an optometrist?  What questions should I be asking?  How
do I do it without offending.

Thank you for your insights.

Chris