Yes, you really do need some sort of spacer where the cabinets meet the wall. This helps prevent issues with the doors not opening to the full 90 degrees, and frankly, it just looks cleaner. A standard 2″ filler strip is typically used to tie the cabinets into the wall. If you have pullout shelves installed, it’s especially important that the door opens to a full 90 degrees—or just under—since there’s a spacer block that keeps the edge of the rollout clear of the hinges.