In the Drill Identification Drawing layer, the program automatically assigns a letter to each of the drill sizes to be exported. It seems to do this randomly. My question, should I be changing these letters to match some IPC drill size standard? I've never done this in 8 years of (occasional) board design work. I've always plotted them as whatever letters the program assigns and no issues. However, I'm wondering if it would be "more" correct to follow some sort of letter-to-drill size standard?
Your Plot Report.txt file assigns the actual drill size to the Dril Ident letters anyway, so your pcb manufacturer should go by that. I therefore doubt that there's a "IPC" standard of assigning Drill sizes, -but I'm not expert in that field.