I am really fast when it comes to PIM triplets in Asturias, but the classical guitar manual says PMI. Why is one preferred over the other? I am working on increasing the speed for PMI nonetheless.
I've always been of the opinion that it depends more on the length of your fingers and the overall construction of your hand. You want to play with the finger pattern that gives you the cleanest, most consistent stylistic sound. It isn't as important whether it's {P – I – M}, {P – M – I} , or even {M – I – P}! I put them in brackets to eliminate any possibility that what I typed could mistaken for anything distasteful. Some people have a finger that is out of proportion to the others, or shorter than the other fingers. You have to discover the pattern that gives you the best service. If you are in doubt, it is always good to at least try what the book suggests, but if you are having more success with your own approach, then use your own approach. Maybe someday soon, we will be using a fingering technique with your name on it. More power to you and Tremolo on!