I'm no expert or veteran on the subject, but I'll reply since carto's are so simple for me.
I always blind squonk my carto until I feel the resistance, then occasionally before I fire up I give a short but sort of strong drag to make sure
juice is nicely up there, then fire it up and vape away.
I do find my thicker DIY juices seem to be the problem with not only wetting the carto nicely, but it also seems to shorten the life of them too. My BWB malty toffee can
juice no problem and the carto's happily go along for the ride. But my vg DIY juices usually kill the carto within a weeks time - no matter how light the color is (if that is the problem - btw, I use PA for flavorings).
So to get away from my ADD and try and answer your question, I squonk every 5 or so puffs depending on the strength of my last squonk, the length of the drags I take and what juice I'm using. The short strong drag before firing may help. I also on occasion do that after a pull or two in between squonks just for the hell of it. But so far I've never needed to turn it upside down to get the carto nicely soaked.