Well, that depends on how old you are. Investing involves the risk of loss. But if you have a 20 year time horizon before you need to use the funds at retirement, you certainly should be investing at least half of your contribution. Consider the Fidelity Equity/Income fund — a conservative fund that should grow over the long run. And leave the rest in their money market fund, so you can relax!
If you’re closer to retirement, put a smaller percentage in the fund. But enough to help your money grow.