censorship and the type of censor (black bar, bubble, mosaic, etc) is purely based on how funny it is to the mangaka. You going for a highly detailed intimidating panel of beowulf with his long john silver out? Mosaic is the only right choice there. A small comedic panel during downtime? That's a black bubble that doesn't fully cover it of course. An epic mid battle erection? You don't want to distract, you want the reader's blood to rush too, so a bright glowing stiffy is the optimal decision.