One thing I do (and I am almost always playing a melee/caster combo) is drive the caster and use the melee like a hunter pet. I set up a macro that cast sequences thru a couple of non-proc based abilities (like Lacerate for druids) and put that macro on all my casting buttons. Then, as Ualaa suggests, I have a separate button for charge and IWT - these are not mapped to anything on the caster. They also happen to be close on my Naga MMO mouse to the assist button.

So its assist/IWT/charge.

tldr its always better if the melee is first to hit the mob, especially with charge or deathgrip to prevent rubber banding.