The client was laboring under a common misperception. RSUs, in fact, are taxed as soon as they vest. Often, employers will hold back an amount of shares equivalent to the tax bill upon vesting. That tax bill is onerous, by the way: Depending on where you live, the Internal Revenue Service, along with your state of residence, could end up taking nearly 50 percent of your RSUs' value. And there's not much to be done about it.
Back to that client: We explained to him that not only did he not have to wait to dip into his vested stock, but that waiting could actually be counterproductive. Should the price of his company's stock fall before he sells, he'd lose twice. First, his shares will have lost value, and second—because RSUs are taxed as soon as they vest—he'll have paid taxes on their higher, original value.
Read MoreCome down from that buyback high
A more common reason that employees hold on to their RSUs is the straightforward hope of growing richer. When I suggested to one 20-something client that he sell his RSUs and invest the proceeds in a diversified portfolio, he basically accused me of being a buzz kill.
"Why would I do that?" he asked. His tech company's stock had been appreciating fast, he explained, and there was no reason to believe it would stop.