Floats generally are not part of a rebuild kit. If they are fuel saturated, they can sink and provide improper fuel levels and must be replaced. It's a good idea to replace them as long as you have the carbs apart.
Float replacement is simply a judgement call. Because they are good today doesn't mean they will be good tomorrow -- or the next. I'll bet the last time you changed spark plugs they weren't really shot either were they? But you replaced them anyway - just to be sure! You'll never have a better chance to replace floats than when cleaning the carbs.
