There are 2 schools of thought on when to "spay" a dog. It is now recommended by 6 months regardless of heat or not. There are valid reasons for this. A vet can tell you the pro's and con's because there are some.
However, I am of the school of thought to wait until AFTER the first heat or first litter. The reason being is the maturity factor and propensity to have weight issues. I've had better results with my animals after their first heat or with my male dog's after a year.
My cat is a prime example. I did decide to NOT wait and got her fixed at about 3 months. It turned out to have been a BIG mistake! I wish soo much I had waited. She has "fallen breast" and they almost drag the ground. People have commented she is a "fat" cat but she is REALLY skinny. It is just her skin is extremely droopy! If you pick her up you can tell immediately she isn't fat. I've also noticed behavioral differences in her. She just never seemed to have "matured" or has that "older" cat mystique. She breaks out in pimples!!! They cover her ears once a year!
It is up to the vet and the individual person on which they chose to do. I believe the trend of spaying or nuetering early is because of alot of animal activist out there trying to get their message across. I agree 100% that animals should be spayed/nuetered but I don't agree with the time line in doing so. I feel the current shorter timeline is out there more as a "preventative" measure. Most people were delaying fixing their pets and by 6 months it was "too late". Having the earlier timeline may help assure the process gets done now rather than later.
Either way you feel... Get the animal fixed!!! It will save a life!