A bad idea. First of all, it takes away from the formality of the wedding outfit. Second, your arms should be really well toned if you are going to do that. I have nothing against sleeveless outfits in general (I wear them myself), but there’s a time and place for everything. Ironically, when my sister was getting married, Maria B told her that she hates when brides go sleeveless because it looks bad. She preferred that they at least have cap sleeves at the very least.