Originally Posted by Duffman

I'm not a financial advisor. Witholding is really just a zero interest loan you make into an escrow account with the government, so you're guaranteed to be able to afford your approximately $20k tax bill every April. You could withhold nothing, take all of that money, invest it into SPY and make 15-30% off of it, but just make sure you can afford the bill in its entirety when it comes due. If weird stuff happens with your pay and you haven't withheld enough, you owe some money when your taxes come due. If you overpaid, you get the excess paid back to you at straight value, without any interest.