http://healthguide.howstuffworks.com/breast-self-exam-picture-a.htm
In my opinion, before menopause.
Why? Before menopause a woman has dense breast tissue. Tumors show up best when they are sitting in fatty tissue, not dense tissue.
Women who have reached menopause have fatty breast tissue; hence a mammogram is more likely to reveal tumors of the breast.
The best defense to fight breast cancer is early discovery. For the pre-menopausal woman that means doing breast self exam.
How do I know this? In my early 40's I had a 'normal' mammogram. About 2 months later I felt a lump in my breast.
It was discounted because I had a normal mammogram. Big mistake.
And my doctor told me a mammogram was definitive. Wrong.
My malignant tumor did not show up on my mammogram because it was sitting in dense tissue.
If you feel you need one, get a mammogram. But don't depend on it alone for diagnosis or discovery if you are still menstruating.
And be advised, you are exposing your breast to radiation, which we are now told is much more harmful than originally thought.
No one really knows for certain how that radiation effects healthy breast tissue long term.
Had I depended on my mammogram results I likely would have had advanced breast cancer before I was diagnosed and treated.
I likely would not be sitting here typing this post. I'd be 6 feet under instead.
Along with the thousands of women that die annually of this disease.
My point: EXAMINE YOUR BREASTS EVERY MONTH.
The life you save may be your own.
Early detection can mean the difference between life and death.
Don't delay. Don't be afraid of what you might find in your breasts.
Be brave. Fight like a girl. Meet the dread disease head on.