I never do this, the reason being is that once my telescope is pointed vertically, I notice the primary mirror tilts back and sits flush in it's clips, and then collimation is off.
I always have it at around 45°. The clips should never be tight holding the primary mirror down. They are just designed to just prevent it from falling out, so they should have a little play.
As an experiment, collimate the telescope horizontally, then slew it vertically and see if collimation holds by testing with the collimation cap. If it does hold, inspect the primary mirror clips to ensure they are not clamping down the primary mirror resulting in pinched optics which is just as bad if not worse than poor collimation.
The only thing to be careful of with this method is the risk of dropping something down the tube!
Regards,
Alan