I highly recommend that you have your kittens spayed or neutered unless you are a professional breeder. Non sterilized male cats may have a propensity to spray (urinate) around your house once they come of age if they feel a need to mark their territory and female cats go into heat quite often and it can get very annoying listening to all the constant howling and behavior that is out of character for your pet. In addition, it is not really fair to the animal to make them suffer through multiple heat cycles with no relief. Do your pet a favor and get them "fixed".
If you opt not to take advantage of my discounted neuter/spay offer check with the humane society in your area, some times they have coupons/incentives that can get the cost down to 25 to 35 dollars. Spayed/neutered pets also tend to have more furry coats and are healthier overall.