Why are you applying filler/spackling in areas that won't be seen anyway? Spackle has no strength, only weight. The last place you would want un-needed weight is at the wing tips to throw off the lateral balance. IMO If you think you need to fill in around around the spars at the tip plate, wait til they are trimmed back and then fill with the Balsarite when you get it. No hurry, remember?