I'd go day by day... the passes there are available in many different combinations. i haven't done the math but my feeling is that the discounts offered for multi-day all mountain might be a false economy.
for example, are you really going to hit all the areas (Annupuri, Village, Hirafu & Hanazono) in a single day every day? you could go for a 8 hour all mountain pass for less, or you might find that you like certain areas better than others or may want to stick in one area all day. you can get a Hirafu & Hanazono pass for less, or an Annupuri only pass for less, etc etc.