Visual design is often last in the pecking order, but i've found it to be crucial to have proper css classes established early on, so even if the precise paddings and hues are not finalized, at least the IDs will be grouped as designed so final tweaks can happen globally and precisely. If 'styling' is left until the end, screens may not be built consistently or as efficiently as desired. A pixel or two here or there or a slightly more recessive grey or blue tone can be a subtle but powerful difference.