As a guy I like the collage without the sashing but I couldn't tell you what he likes. You have to ask him. Lay out a couple of setups for him to choose from and then do it that way regardless of how you feel about it. After all if it was yours you would have flowers on it. ( Exaggeration there just to make the point).
Comics are about action and the panels are deliberatly complicated with a lot of detail. Sashing would take some of that away and loose the flavor to my way of thinking. You just gotta ask him how he wants it.