Halachic consensus is that a woman must perform at least one bedikah on each onat perishah.
It bears noting, however, that there is a range of perspectives on when to observe onot perishah when on the pill. We hold that a woman observes onot perishah beginning with her minimum interval from cessation of active pills to the onset of bleeding. For more details, please see our site's article, "Vesatot and Hormonal Contraception."
You may also wish to contact your physician about the possibility of changing pill formulations, which may lead to a decreased incidence of spotting.
Please write back with any further questions.