Being a Litvak, I was taught that you don’t daven to flesh and blood, not your favorite Tzaddik and not Rachel Imeinu. You daven to HKB”H. Period. No one else. You can ask HKB”H that in the merit of so-and-so you will be answered, but that’s not davening TO the tzaddik.
I used to get into discussions with my Catholic friends about how they could pray to the statues of saints, who were after all human. I never got a logical answer. The idea was to pray to the saint to intercede for the person, but that’s still praying to a human being.
Davening TO a tzaddik seems to me to be to be not quite kosher. If you visit the kever to help you concentrate, fine, but not to daven to the tzaddik him/herself.