It does appear that if the stem fits tightly enough in the hole in the washer that it won't be the rubber supporting any tension from the nut, since the metal itself flares out slightly at the base of the threaded or non-threaded stem.

The way that rims support the Presta valve stem base and tube varies quite a bit when double-walled rims are added in, some of these have wildly-varying hole-size differences in the "inner" wall even as the visible "outer" wall is clearly sized for the Presta stem. The bigger hole allows the rubber reinforcement and the bigger end of the stem to settle in and thus better support the wall of the tube evenly right to the valve stem attachment. But only when both such holes are small will it prevent the stem from rocking too much while pumping.

I still know of no perfect solution, or what is really best, but Shraeder valves are generally trouble free and durable, with or without any threading. Thankfully, today's wider rim profiles allow more room for the reinforced rubber attached to the base of a Shraeder valve stem, even as perhaps fewer of them come drilled that way unless targeted to low-end applications. To that I say "three cheers for low-end rims",