You'll be lucky to get resistors with a wider resistance tolerance than 5% today, however electrolytic caps with a capacitance tolerance of anything as tight as 20% would be rather unusual.

As Treewind pointed out, the OP's question refered to operating voltages rather than component values.

I've run valves at double the maximum recommended anode voltage without a problem....electrolytics wouldn't be quite so forgiving!

Interestingly I've also run valves at low voltages (20-30v) and they've worked remarkably well in signal amplification applications, I've not used them in power applications though.

I'm going to get a beer.

JJ