Investing for retirement is a long-term proposition. That means investors can take on more risk as their investments have a longer period of time to recover from any market volatility.

But, even with 40 years or more to invest, not everyone is comfortable watching the value of their retirement account go up and down.

Investing conservatively is not without risk either. Giving up the possibility of higher returns is an opportunity cost that could result in less money at retirement.

As there's only a finite amount of time and money for most people, meeting retirement goals may require compromise.

"If somebody is going to run their plan out at 4.5 percent and it shows they're on pace to get 70 percent of their retirement objective, then they need to either save more money, work longer or retire on less," says Larry Rosenthal, Certified Financial Planner and president of Financial Planning Services in Manassas, Va. "Or take more risk. Those are the four choices. Usually it ends up being a combination of all four."