If you have a fully fueled ferry at Minmus then yes, eject straight from there to interplanetary. While a burn from low Kerbin periapsis is more efficient, you would have to burn a good amount of fuel to reduce your periapsis from Minmus's orbit down to that point.
All of my interplanetary missions leave from Minmus orbit, in fact for a while I maintained a fueling station there ("Minmus Outbound") that I kept filled by drone "ore piggies" from the surface. And when I returned from Duna and Eve, I did so by topping off the tanks at Ike and Gilly respectively. Will do the same from Pol on my way out of the Jool system.
Mister Dilsby's post in Depart For Duna was marked as the answer