I don't recall where I saw the statistics, but it's rare for a cat to spray after being altered, especially while quite young. Jamie was neutered at six months, which is average in Germany, and started to spray when he was three and a half. He sprays the perimeter of our property every day in several locations, and the spots alternate, so his scent remains fresh. The scent is pretty much odorless to humans, unlike that of tomcats.
I believe there were a couple of factors that started the behavior. About the time he "learned" to spray, there was an intact male cat in the neighborhood who insisted on marking in our yard. Our tenants had a male dog who lifted his leg around the perimeter of our property, and he and Jamie were buddies, so Jamie may have been copycatting. Our yard has always been the favorite hangout for neighborhood cats due to its bird feeders, catnip and availability of rodent prey (lots of shrews, mice and rats), which has led Jamie to warn off intruders. I also feed a tame hedgehog catfood outdoors during mild weather, so some cats come for a free meal.
Jamie is an indoor cat who goes for a supervised walk wearing a harness and leash every day, and that's when he does the spraying. It isn't a problem inside the house.
He and another neighborhood cat, also neutered, got into a spraying contest on either side of a cellar window for a time, and we stopped that by putting basil plants around the window, inside and out. That was a tip from a breeder who accidentally discovered that fresh basil prevented one of her queens (i.e., intact female used for breeding) from spraying.
We obviously could have done a lot to stop Jamie's marking behavior by discouraging other cats from hanging around here, but don't view the spraying outdoors as a problem. Something as simple as using Feliway spray or a diffuser may have worked.
Our last cat was a former feral who was eight and intact when he started living here, and it took us around a year to get him to the point where he could be castrated (traps didn't work). We gradually moved him indoors, and he never sprayed inside the house.
In short, while some neutered cats will spray, it's probably due to special circumstances, and not something you ordinarily would have to worry about.