There are a lot of stereotypes like that out there, but IMHO gender has less impact than breed, breeding, or environment. Are females less boistrous because estrogen has an impact on behavior, or because people treat female dogs differently than male dogs in their formative months? I think the latter. Now I have seen pretty consistent behavioral traits related to breed, for example about 80-90% of JRTs are, shall we say, assertive with other dogs.
If you're going to get a purebred pup, or even a yearling, spend lots of time with the candidate and use your judgement. An ounce of direct observation beats a hundredweight of old wives tales.