We were told that spaying before the first season gave the maximum protection against certain cancers (breast cancer, maybe?). Basically, the vet told us that you lost a quarter of the protection with each season the dog had - so if you spayed after one, it would be down to 75%, after two, 50%, and after 3, 25%.
That's not to say that spaying gives 100% protection against this particular cancer, but that it does give some protection, and if you spay before the first season, you get 100% of the benefit that spaying offers in this regard.
We did spay our chocolate lab before her first season - and she did develop a bit of urinary incontinence - as a result of the spaying, we were told, not as a result of when she was spayed. Apparently spaying can cause a hormonal issue where the urine becomes very dilute, and the dog occasionally has an involuntary wee - we refer to it as having an occasional leak.
There are hormone tablets the dog can take, that tightens things up for a while - our dog has them from time to time, and they do seem to help. But because the urine is so dilute, it is not a huge issue for us - it doesn't ever leave a residual smell or stain behind, and is more like cleaning up spilled water than wee.