I'm interested in buying a reel of Wilson stamina 17 (660' ) instead of buying individual packs of string... How many racquets can I string with a reel? I'm guessing approx. 18, Is that correct? If I don't string racquets often (maybe 2 or 3 frames per month), is it worth getting the whole reel? Will the string stay "fresh" if I use the reel of string as opposed to individually sealed packs? WIll this "freshness" make an impact on playability/durability?? Thanks.