This is very similar to what Ms. Loonie is doing by saving up her tax payment rather than having income tax withheld by her employer. I really like this idea of paying yourself rather than giving the store an interest free loan (at best) or an outright gift (at worst).
Obviously, the success of this approach hinges on two things:
- You only use the "warranty" account to repair or replace defective items
- The expected failure rate of the item in question is low enough that declining the warranty is a "good bet"