It depends somewhat on your individual conformation. The key point is the correct grip in the calf. For some, the 45 degree angle rolls their knee out and puts the grip in the back of the calt. For others, folks who naturally toe out a little, can maintain the 45 degree and the correct, soft, wrapped grip in the flat of the calf.
Forcing the toe forward can also roll your knee too far in and lift your whole leg off the horse.
This is one of the reasons GM advocates placing the stirrup at an angle across your foot, with the outside branch on the pinky toe - it kind of splits the differnence and allows for the variation in individual's conformation.
Even though it seems arbitrary at times, these equitation "rules" have a basis in function. If you have the correct grip in your lower leg, then your toe angle is correct. If you don't have correct grip, adjusting your toe angle is a good place to look for the cause.