No right or wrong answer, they don't "need" a mic, as they pretty much come with the effect of the mic rolled in, just as the speaker cab, speakers etc all affect the tone it produces, HOWEVER, you can still add one easily enough and that is basically like just adding more EQ to the IR (mic sims actually are IR files I believe)
So instead of using some EQ, give it a try with a mic sim, might cut/boost certain frequencies and give an effect your after.
In the end its really just all about how it sounds. Doesn't matter if your using a neutral IR and then some PEQ etc, or if your taking a different IR, adding mic sims, filter blocks etc.