The trouble I have with the idea of heifer bulls is that they are taken on into the bull battery and continued being used on cows. If you have good heifers they should be able to have good calves and continue good average genetics. I calve mostly out and choose bulls that are in the 3.0 epd birthweight average. I rarely have to assist in calving other than when there is a leg back. One of the worst years I had was when I calved a bunch of heifers bred  to a mytty in focus bull. I had to pull close to 30% but felt it was more a problem with not being able to have them out on pasture where they could get lots of exercise than it was with the bull. Maybe management of heifers is almost as important as the bulls you use sometimes.