Grafting Fruit Trees (Citrus)?

When one grafts, or "buds", citrus tress or any tress for that matter, don't they have to ensure the rootstock is female while the scion/cultivar is male? Otherwise you could (unknowingly) be crossing same plants?! I'm confused. Please clear this up for me. I want to graft soon. I have - Loquat, Tangerine, Tangelo, Meyer Lemon, ....