What lvl are you? If you have the 53 song, you can reliably mez 3 mobs.
With Pixie Strike, you can only reliably mez 2 mobs. I would never mix
charming and mezzing and a single bash can really ruin your mezzing.

For me if there is a 2 pull, I will usually charm (though in Sebillis I now
just mez, since too many of th emobs are over lvl 50).
If there is a 3 or 4 pull I mez. If you are getting 5 pulls you better have
an enchanter or a real good rooter...