Good idea? Yes (in many situations).
Most decent headunits have 2V-8V preouts (most being in the 2-4V range), where-as most soundcards have 100mV-1V preouts. What that equates to is having to drive your gains up higher which can shorten the life of your amps and also introduce distortion. The problem really is getting away from cards that have such low voltages though. Putting a device in between that bumps up the levels is a band-aid solution. Your best bet is going for high quality sound cards that have higher outputs so you don't need to "fix" the problem. EQ's do have benefits in their own right and are used for those properties. I'm not saying not to use one, but don't use one for your particular concern unless you absolutely need to.