jeez, I'd be inclined to stick with spacers over washers. The wheels have a large mounting face for a reason, and the spacers are sufficiently large for the same reason: to evenly distribute forces from the wheel to the hub and to evenly distribute the forces of tightening the nuts over the wheel and the hub.
If it was the "done thing" you can be sure you could buy washers in the aftermarket and they'd all be doing it.
I realise you might have proven to yourself that the washers work but for peace of mind I'd pay out the tenner or whatever the spacers cost.
I get a pic of the "washers" i picked up last year