Actually, I'm a Navy trained avionic tech that works in civilian aviation. With that said, when someone says to "go ahead, it will handle it", that's asking for trouble. Same deal with running a car on regular or E-85 gas that says only use premium unleaded in the owners manual or running 5-30 in an engine that says 10-40 because some hack says it's fine. Either one of those scenarios will result in a damaged engine and major amounts of cash to repair it. If the servo says use a specific voltage, use it. You are correct, there is a fudge factor, based on how much the batteries will hold at peak charge. A 6 volt nicad pack will probably top out at 6.5 or so, about where a LiFe pack is rated. You put a fully charged LiFe pack on a meter, it's probably over 7 volts. That will fry a 6 volt servo in short order. And, just for the record, I don't know of anyone under the age of 75 that drives the speed limit. If you try to drive 60mph on the freeway in my area, some one will end up dead