There's been some discussion off and on elsewhere regarding sticking religiously to the NMRA recommended weight for HO cars. There's a segment out there that's of the mind that it's not as much the weight the NMRA states, as it is that all the cars weigh about the same. Consistency is the key for those folks; some of them have cars that are an ounce or two below what the NMRA states.
But, if you run at the club, or have friends who contribute cars to your sessions, then having everybody adhering to a standard helps keep things running smoothly. If you're a lone wolf, then you could save a penny or three by not filling your cars with weight.
Kennedy