I don't group items in my shop. I don't know if it helps or hurts. I like the way it looks in some shops (especially when it's done by color - it looks very cohesive), but personally, I like to get a feel for everything the shop has, then use sections to narrow things down. I like to arrange my shop so that there's a good mixture of the items I have for sale on the first page without having too many of the same type of items right next to each other. I forget to arrange it a lot, so when I renew a lot of sold items it can get a bit messy.