You may need more (or less) money than you thought
Blindly following a rule of thumb can backfire in retirement. For instance, it's commonly said that retirees will need about 80 percent of pre-retirement income, but that's not always true.
"People may choose to travel quite a bit, which means they are spending more than they were (before retirement)," Munn says. "And others have such simple lives that they may spend far less than they ever thought they would -- especially when you consider that they may have paid off their mortgage shortly before retirement."
Similarly, withdrawing 4 percent of your portfolio simply because you heard it was the safe rate of withdrawal can be a recipe for disaster.
"A lot depends on how they have their money invested. If someone just has CDs, for example, they are earning very little. If you factor in inflation to expenses, if they start at 4 percent, a few years down the road they are at 6 or 7 percent, and their portfolio is going to be depleted very quickly," says Munn.