I've been around a lot of dogs over the years, and from my experience dogs are aggressive (to people) for two main reasons (barring health issues):
1. Improper handling/poor socialization/combination of the two
or
2. They were born nuts.
The vast majority of dogs, either altered or unaltered are not dangerous or truly aggressive to people at all. As to the other question of the OP, I can't really say whether I've noticed any behavioral changes in dogs after they were altered, because I might know them for a week or two before FOHA takes them to be altered, and I also know them in a kennel environment, so these aren't really ideal circumstances for gauging behavioral differences.
Daisy was spayed at 6 months, and Benji was neutered fairly soon after we obtained him, and Rocky was already neutered when my mom got him from CL, so I don't even have reference from personal dogs, I'm afraid.