Hello there,
Thought I'd jump in on this one too. Being a operating room nurse, I can definitely see both sides of this issue. USUALLY, these types of surgeries have very little blood loss, and you actually have to lose a certain amount before a doctor will choose to trasfuse you blood anyhow. Of course, some people have ran into instances where they have needed it, and it's always a nice thought to be able to have your own blood.
On the other hand, it is also nice to keep that "pint" of blood inside you if you can, too. Women have less blood in their systems then men, and sometimes it helps to be at full-force when it comes time to surgery and recovery. If you auto-donate, just give your body enough time to make up that "pint" again. Your physician can help you determine timing.
Keep in mind that using donor blood is VERY safe nowadays. Where I work, I often hear that all-to-common phrase that it was 10 times riskier driving here this morning than it would be to ever catch a disease from someone else's donated blood, etc, etc.
Of course, only you can make the right decision for yourself. Do whatever makes you feel better about the day of surgery. We all know too well what goes through your head, and any decisions you can make that lessens the stress for you, do it!
Best of luck,
Amy