I've heard importing a header to Prefix.pch can improve performance as it will load the header once, instead of on each instance. So when is a header imported enough times in a project to warrant being included in the Prefix.pch? A dozen times? Several times? Just twice?
I typically add a header to Prefix.pch if it is being #imported into more than 3 classes. I don't think there's a rule of thumb - it depends on your own standards for what you consider clutter, elegance, how globally the code is used etc. For example, most of my projects have a Utilities.h and Utilities.m where I declare helper functions used in many places. I always add Utilities.h to my .pch.