How Often are Routine Doctor Visits Recommended: OB/GYN, Dentist, Optometrist/Opthamologist, Dermatologist, and others?

I know that I am suppsed to visit my OB/GYN once/year for a Pap Smear and to get a refill on my birth control pills. I also know that I am supposed to go to my dentist/dental hygienist every 6 months for a dental cleaning and checkup.

I am wondering if there is a list of recommended preventative screenings or "check ups" that we are supposed to get, for certain ages (from an unbiased source)? (For instance, is my dentist wanting to make money from seeing me twice a year, or is this medically necessary)?

I am not sure if I should see an eye doctor (optometrist or ophthalmologist) if I am not having eye problems: is there a recommendation for checking vision on a regular basis? What about seeing a dermatologist: should we have our skin checked "just because", even if we are not showing any signs or symptoms?

Lastly, any other recommendations for doctors/speaclists that we should check into BEFORE we have problems? What is good prevention, anyways?

Thanks!