Its a good idea! BUT.....doing the numbers:

Walmart 6% bleach is $1.67/gallon. $1.67*55 = $92.00. Double it (for 12%) and you get $184.00.

So you would be saving $34.00, but having to store 55 gallons somewhere (all at once, instead of buying it when you need it), and also need to factor in the fact that 12% turns to 10%, 9%, etc. faster than 6% turns to 5%, etc.

Every year I try to think of a better way, and every year I just end up buying jugs from Walmart.....usually enough to cover 2 weeks at a time, just to save trips. Oddly enough, this year bleach hasnt gone up in price from last year and the container size didnt change (like they would do to "sneak in" a price increase without actually increasing the price)