Yeah its just preference as said above, but IMO you should always use the default skin for hitsounding and selecting combo colours. Hitsounds because not everyones going to be using custom skins or the same custom skin, so hitsounds might end up sounding really weird without that specific skin. Combo colours because some skins have stuff in the middle of the hitcircle and some have nothing to actually colour, so picking the right colours (That look good for all skins) for the map ends up being very hard