Unless there is a specific need to ground the negative side of a DC power supply, and I'm referring to 24v units in this post, I try really hard to ensure that there ISN'T a connection between DC- and earth ground. Depending on what you're powering with that PS, the loads may actually REQUIRE a floating supply. Sometimes, especially with PS's that supply 0-10v or 4-20ma analog circuits, grounding the DC- actually introduces unacceptable noise into the system.
When you ground the DC-, you also loose your isolation between input and output. The whole point of a switchmode power supply is that it gives you that isolation. In fact, I believe UL508A requires you to NOT ground the DC- of your power supplies for all of the above reasons. The inspector should quote you chapter and verse of where he believes it to be required.
SceneryDriver