rebuilding a trans question on parts

soon ill be rebuilding a 904 trans to go into my demon.. looking at the rebuild kit and bands i can get bands and discs that are smooth or grooved.. what the benefit of one over the other? also how does the reverse wound spring make a difference then a standard spring in the direct. i was also told that i can tighten up the clearance in the direct more then what the books states.. any opinion on this..