I just stick it in until I can feel it with the other hand then manipulate the cervix/reprotract.
To me it depends on how easy or small the cervix is especially in the case of young heifers. I use the right hand to thread the gun through the cervix as much as I use the left to manipulate the cervix. I AI'd lots of jersey heifers a few years back and they were small, on the low end they weighed just over 500lbs. That's probably where I started doing it this way.