If you follow my discussion of sales tax deductability (here, here, here and here), you might ask one more question: as tax filers can choose to use IRS's table to claim state sales tax deduction, or save all receipts and claim the actual sales tax paid, should one start to save all receipts for a chance to come ahead of the game?

The answer to this question will require some analysis, and as everyone's situation is different, be sure to run and check your math before investing your leisure time in the receipt-collecting exercise. Of course, it does not hurt if you check how I frame this:

I started by projecting my household's adjusted gross income (AGI) for next year. After deducting 401(k) contribution and flexible spending account enrollment, I feel we will end up with a bit more than $140,000.

From Publication 600, an income level of $140,000 will amount to a deduction of $1,277 for a family of three in the state of Washington. Now, we need to lift it up by converting from a state sales tax of 6.5% to an effective tax of 8.8% (adding local tax, etc.), which means without saving all receipts, I can expect to claim $1,728 deductions next year. At an estimated tax bracket of 28%, the true value of the deduction is about $484.

The next question is: how much of my spending next year will be subject to sales tax. To answer this question, I need to look into my 2004 spending and make a projection. My estimate for 2005 is:

- Auto Maintenance: $1,200
- Home Maintenance: $1,500
- Clothing/Toy: $3,000
- Dining Out: $4,500
- Groceries: $3,500
- Major Appliance/Home Furnishing: $4,000
- Local Entertainment: $500
- Other Expenses: $3,000

Note the above list does not include:

- Food and some other groceries that are not subject to sales tax
- Other expenses not subject to sales tax (like daycare, gas, insurance premium, utility, mortgage interest, etc.)
- I also take out some out-of-state expenses (like vacation, online shopping, etc.)

The above budget, which I feel leaves lots of buffer, adds up to $21,200. At 8.8% tax rate, It should represent total state sales tax payment of $21,200 * (1 / (1 - 8.8%) - 1) = $1,969, which is about $250 higher than the amount I can deduct using the default table. At 28% marginal tax rate, it makes a difference of about $70.

So, the fundamental question is more like: is it worth to spend hours and hours to save all receipts and track sales tax for the potential $70 gain?

Nope for me. I value my leisure time at about $25 per hour, so I'd rather spend more time with the family than playing with more numbers. But again, my conclusion does not automatically apply to you. Especially, if you are a big spender, probably you can come ahead by preserving a record for your actual sales tax payment for next year. Make sure you run your own math before coming to the conclusion.