Question:
     I was always told a cat or dog should go through at least one heat cycle before they got spayed otherwise they would never be contented, happy pets.  Is this true?

Answer:     cat... when to spay... how old
     No.  It is not true that cats or dogs "should" have a heat cycle prior to being spayed.  The only proven undesirable effect of spaying or neutering at any age (other than being unable to have offspring, which for most people is an advantage) is a decline in total daily calorie needs.  That simply means that you can feed your cat or dog less food after being spayed or neutered.
     The "disadvantage" is that if you continue feeding the same amounts of food after the spay or neuter procedure over time the pet will almost certainly become overweight.  The fact is, though, that cats and dogs cannot prepared their own meals so you, the pet's owner, have the ultimate responsibility for your cat or dog's body weight.
     Every day we veterinarians hear well-meaning people state emphatically that their cat or dog "got overweight after she was spayed"... as if the surgery created the weight problem and the amount of food being eaten is unimportant!  This is a classic cop out to deflect responsibility for the pet's body condition to something other than ourselves.