Quite a few people I do pruning and landscape work for, bring up their lawn conditions and point out weed grasses. Although weed grass seed can blow into yards in some areas, I find that in dense urban areas, not much moves-in via wind. I told the people that their weed grass problems can be reduced if they request their mowing companies to wash their equipment between properties. Its not practical for the mowers, but its truly one of the more professional techniques to protect nice lawns from being infiltrated with weed grass seeds. Unless every lawn on a route is nearly weed grass free. But that's often not the case.