I don't worry about this at all. Maybe my physics understanding is off, but once the bearing balls are in full contact with the races, it shouldn't matter whether there's a little or a lot of preload. The grease is going to get displaced along the path of ball/race contact, period. The issue that concerns me is how quickly the displaced grease can migrate its way back into the contact path. This characteristic is determined by viscosity, I suppose. I forgot whether it was Jobst Brandt or Mark Stonich who posed the argument that for bearing surfaces such as headsets, BBs, hubs, etc, oil is actually superior to grease for exactly this reason, but that grease has replaced oil simply because of the convenience of not requiring as much maintenance-- with oil the same characteristic that makes it advantageous to bearings also causes it to spread and drip outside of the bearing assembly and onto other parts of the bike.