NEW YORK (AP) — Coming up with the best tax strategy in retirement can be much trickier than it seems, and tax pros agree it’s a time when people need to be especially careful to look at their entire financial picture before deciding on things like 401(k) withdrawal amounts and timing, or when to begin taking Social Security.

“It’s the biggest change in life other than death. Don’t be so quick about deciding when to take Social Security benefits or 401 benefits. Talk to a tax professional before you make these decisions to avoid surprises. It may save you a lot in tax dollars,” says Tom O’Saben, director of tax content and government relations at the National Association of Tax Professionals.

Withdrawing a large amount to do something like pay off a mortgage could result in a dramatic increase in what percentage of your Social Security benefits will be taxed, for example.