I dove into the PMT around mid-grade 2. I had questions like “why are these notes in a chord” that were quickly answered in PMT. Doing PMT earlier would probably have given those answers to you before you needed to ask them.
Later, knowing some theory will help with transposing, figuring out triads, following various scales, and that sort of thing. A recent great practical example is the play notes Shane has in this link below. Read the notes and you can see how understanding theory helped him understand why the song was set up the way it is.