The S&P 500 is still a smart all-around option, but there's good reason to start thinking about other index funds.There's never really a bad time to buy the Vanguard S&P 500 ETF (VOO 0.60%). But there are better times than others: It's better to buy on a dip than at a peak, for instance. If your goal is to simply plug into the broad market's long-term performance, though, your best bet is just stepping in whenever you have some otherwise idle cash you're looking to put to work.Every now and then, though, it ...