I'm fairly certain that it helps. Obviously someone has to start, but when it looks like no one cares others are more likely to contribute to the problem or worse assume that leaving trash there is what's expected of them. The Cart Narc guy has observed a similar trend with shopping carts. If somebody puts one where it doesn't belong it can attract others. You'd think that if people were going to be lazy and leave their carts in the parking lot instead of returning them properly they'd just leave them near their own cars, but some people will go out of their way to put theirs next to other carts even when it's still clearly not where they belong.