For me, I like to relate all twisting skills in the same direction in order to reduce complications for future development of higher level skills. Very often, this relationship deals with familiarity and skill similarities.
(I aim to do this with all skills with the exception of the roundoff)
For instance, if one is looking for exploring giants with turns and pirouettes and which direction to choose - you can relate a Giant to a Backwards Layout on the floor..
Sure, there are clear differences, i.e. attachment of the hands to a bar and being in free space.. However, the fact remains, a gymnast is rotating 360 degrees.
If we use this same process to evaluate a Giant with 360 turn in handstand, the skill similarity is related to a Full Twisting Backward Layout on Floor Exercise.
This relationship creates a familiarity of movement patterns for the gymnast that hopefully will aid in performance reliability.
All this being said, I base turning direction on Bars to the turning direction the gymnasts assume whilst performing lateral axis rotations on the Floor Exercise/Trampoline:
If a gymnast is a left twister during lateral axis rotation, they will blind change by releasing their right hand (posting body weight on left hand) and pirouette by releasing the left hand (posting body weight on right hand).
If right --> Opposite of above...
For me, this template creates a solid foundation of familiarity and skill similarity.
I do believe however, that the argument of allowing gymnasts to do what they find comfortable, does have merit. If a coach allows this, they must be quite knowledgeable in skill selection and problem solving should a directional complication occur in future development and maturation of skills.
I would teach both pirouettes... I normally do this for three reasons:
1. Routine construction... Being able to move either way in a pirouette, or top, can decrease shuffling and deductions for lateral movement.
2. When spotting and learning blinds, the gymnast can top back into me and avoid shuffling back towards me after pirouetting away from me.
3. Creates a symmetry in support strength on one arm.
Hope that helps!