A study by the Urban Institute has found that 401(k) employer contributions aren't exactly "free." While traditional pension plans were seen as a benefit on top of employee salary, today's employers factor 401(k) contributions into the overall salary equation.

The study compared wages at employers who contributed to 401(k) plans versus pay at employers who didn't. Those who received 401(k) contributions tended to have lower salaries than employees at companies where no contributions were given. And for many employees, the salary dip was roughly equal to the potential 401(k) employer contribution.

Low-wage workers actually make out a little better, as their salaries dip less dramatically than higher-income earners when employers make 401(k) contributions. Low-wage workers might be less able to put money aside, though, making it less likely that they'll take full advantage of employer matches.