No, this isn't the case. Generally speaking, ovulation occurs primarily at a point in the cycle that is quite favorable to implantation---when the uterine wall is well-prepared. So the primary effect of the rhythm method is to prevent conception, not implantation.
I do believe that it is possible for a woman to ovulate outside of that time frame on occasion. I've heard it can be in response to some stimulus (like sex) or spontaneous. I guess in that case there is a lower chance of implanation. But someone practicing the rhythm method isn't necessarily encouraging this phenomenon any more than someone who is not.
Someone please correct me if I'm wrong.
Hmmm... you're probably right. It's been ages since I've had sex ed, and with no wife I've no reason to worry about such things. Though I don't remember whether the ovum has necessarily lost all viability before the body begins the process of menstruation. Oh well, it's late and I'm tired. 'Night.