I am currently also weighing the pros and cons of spaying. I am leaning towards getting it done. My puppy is 4 days away from her 6 month Birthday. I have heard that it is better to spay them before their first season, but I have also heard that it is better to wait until she has had one.
We are going home for Christmas and will be surrounded by 3 of her brothers and they will be primarily outside dogs (with no fence, in the middle of no where) for the holidays. Seeing as she (well, they will all be) 6 going on 7 months old, she could come into season at any time. So I am thinking getting it done before hand might be a prevenitive method.
Drai

The reason most vets prefer to wait until after the dog has had her season is because the reproductive organs become encourged with blood prior to a season, and by speying AFTER the first season they stand a better chance of not ending up operating on a dog who is just about to start a season, and therefore reduce the risk of haemorage......

The womb actually becomes engorged during a season, but you are right about it being dangerous to spay through a season. Most vets nowadays prefer to spay at 5 months or 3 months after the first season (as sometimes bitches come into season early and cant be spayed at 5 months). Any good vet will be able to examine and assess your girl to ensure she isnt in season prior to surgery, so I would discuss getting her spayed with your vet while she isnt showing signs of oestrus. :)