In retirement, a good adviser can help you withdraw money from investments with minimal damage to your overall portfolio. That person should also pay attention to the tax implications of purchases and sales to avoid a big tax bill.

"If you're not responsible about how and why you trade," says Mervine, "it can really limit your returns. Everything you pay out in taxes over time can be very dramatic: tens of thousands, if not hundreds of thousands, in a lifetime."

Then, of course, there's the risk of changes to the tax brackets or the tax code generally. "We are aggressively converting from IRAs to Roth IRAs because that would immunize our clients against tax risk, which I think is going to be substantial," Reese says.