I have always spayed at six months before the first season and will continue to do so (large breed dogs). This is also the view of the majority of the vets at our practice - there's just a couple who would advise leaving larger breeds until after a first/second season (and one who breeds and leaves his bitches entire for life).
There's no right or wrong, there's pros and cons either way, just read everything available to you and make the decision you feel is right.
6 months is usually about right for most dog breeds, I think some vets will recommend waiting a bit longer for giant breeds or miniature breeds to make sure they are developed and strong enough to get through what is a major operation.
I would have it done at 6 months, before any season.
I have a male pup, so isn't entirely relevant, however my best friend has a young bitch who came into season just before they were going to get her spayed. She's now going through a phantom pregnancy, and there's a slight risk of her coming back into season before she's eligible to go in for her spay. It's a lot of hassle for the owners, a lot of hormones for the bitch to deal with and there are the associated increase in health risks with each season.
Different vets say different things, and different owners say different things. Also show dog owners and breeders will have different opinions based on their doggie experience, so I don't think there's a wrong answer. Just weigh up the pro's and con's of early and late spay and see what works for you.