RE: Baitcaster ?
overspooling the reel with too much line causes backsash....like chuck said leave it about an 1/8 inch from the tippi top.
when you spool your line hold the Rod infront of the reel while holding the line between your thumb and forefinger (to put some tension onthe line)while you crank with the other hand.
or you can use a vice with a screwdriver....Put the spool on the screwdriver and then throw a towel over the Spool or line(to ad tention....and also back in the day some mono line had a chalky film on it, using the towel would get rid of that.)
Fresh mono is the best casting line IMO...when you get over the 20 LB test it starts to not cast so good because of the stiffness/diameter. mono in the 12-17 lb range is ideal all purpose line. amd casts great