Financial experts often tout the three-pronged tax benefits of health savings accounts. In order to maximize those tax benefits, financial advisors often make a recommendation along these lines: Pay out-of-pocket for today's medical bills — rather than tapping the HSA immediately — if you can afford it. Meanwhile, invest your HSA funds in the stock market to build up a potent tax-advantaged war chest . Then use those proceeds decades in the future to cover past out-of-pocket health costs — completely tax-free . They can even be used to pay yourself back for medical expenses incurred years prior. But this advice is somewhat incomplete: To reimburse yourself later, you also need to save your receipts and other documentation for that old medical expense — or risk incurring taxes and penalties from the Internal Revenue Service during an audit, according to financial advisors.…