I'd say Old Wives' tale, because I had TERRIBLE cramps and was not incredibly fertile. It took 18 months to conceive #1, 8 for #2 and 10 for #3.
I am on the Pill, because it lessens my cramps and my flow greatly. Plus it's sort of in my hands. If I get pregnant again, it's my fault, it's not because some Dr. didn't tie my tubes or my husband's right, or didn't place the IUD in right. As long as I use it correctly, I know I won't get pregnant.
Although, after 3 kids, I'm toying with the idea of both me AND DH getting fixed and STILL staying on the Pill. Just to be doubly- triply- extra sure.