I use foliar if I spot a problem, but typically (in soil) I'd add in 1/4-1/2 dose of calmag in every 9th watering or so as a pro-active measure. My routine in soil is feed, feed, water. feed feed water. feed, feed, water w/calmag.
I'm just starting a (my first) perpetual Hempy grow, so I think after the plants are ready for nutes, I'll stick to the same schedule unless the plants warrant something different.
-spek