Cover dates are useful for organizing and research, but I always go by newsstand arrival dates to classify books. For example, December of 1962 had a slew of great marvels (JIM 89, TOS 39, FF12, ASM 1) on the stands. These are all cover dated March, 1963 (except JIM 89 - Feb 63) but when I look at them they are the last of the 1962 marvels in my mind.
Another major division for early Marvels are when the Ditko based “side bar” started in May, 1963 books (Feb 63 newsstand time - ASM 2, FF14) This is when the 12 cent bubble disappeared and was replaced by the side bar art and price boxes.