With donated clothes/ furniture/ etc., you can deduct the fair market value of the item in its current condition (i.e. used), no matter what you paid for it when it was new. Using the same line of thinking, you can probably logically justify a deduction of the fair market value of food items (which are not used, and thus not reduced in value from the retail purchase price), regardless of what you paid for the item.
However, in an audit, the IRS may come to a different conclusion if they can show that you did not pay fair market value for the items — i.e. if you make $60K a year, and are deducting $25K of donated groceries because you got them for free (virtually), the IRS is going to send up red flags and start asking questions that are hard to answer.
I would think that if you are making reasonbly-sized honest charitable deductions, you should be fine ... if you are using this as a tax shelter, the IRS is likely going to smoke you.
SnakeDoc
Well, evidentally the Coupon Queen who is teaching this precise method in her Coupon Classes claims that she saves several thousand dollars in taxes each year.