When should I perform the inclination matching maneuver?

Inclination matching can only be done on the ascending or descending node. At any other point of the orbit you can not achieve perfect inclination matching.

While most course corrections are done best when you are fast, i.e. near periapsis, it's the opposite with inclination. The slower you are, the less delta-v you need to change it. So when one node is on a higher altitude than the other, you should do inclination-matching at the An/Dn node that is higher above Kerbin than the other one.

When you want to minmax your delta-v budget, time your launch until the rotation of Kerbin turned the space center to be directly beneath the ascending or descending node of Minmus' orbit around Kerbin thus making perfect inclination matching (see above) possible. Then head for the inclination during the gravity turn, i.e. aim for heading 96° instead of 90° in regular lift-off. That way your low-Kerbin parking orbit will be already inclined to Minmus' orbit.