Let's say you're 5 boxing.....

Pay for 6 months on all of your accounts (this can be tricky getting it set up to do this correctlyl - maybe the blizzard billing department would help you).

January - Pay for account A - 6 month sub (lowest price per month)
February - Pay for account B
March - Pay for account C
April - Pay for account D
May - Pay for account E
June - "free" month wherein none of your accounts require a payment.

Or you could just prepay the year on all of your accounts with your tax refund.... Kinda like prepaying your car insurance for the year to save overall.