Definitely not. For one thing, every heat cycle brings a significant risk of pregnancy—and first cycles occur as early as four months of age in kittens, six months of age in puppies—dangerously young to have a litter. Spaying before having a single litter greatly reduces the risk of certain cancers. Besides, we already have a staggering surplus of kittens and puppies literally dying for want of homes. Every single litter adds to the problem.