[Arran]

Generally it's recommended that you go slightly higher in value if you can't get an exact value like with a power resistor. In some cases it wouldn't hurt to go bellow because the actual value will be above the marked value and close to what the schematic specifies like replacing a 500K ohm resistor with a 470K ohm. I would not go with + or - 20% though, that's too much. There are standard values of 5 and 10% tollerance, 5% is the most common, there are also ones with a 1% tollerance and precision resistors.
I've made a habit of replacing any resitor more then 10% off in value while overhauling a radio, when 1/2 watt 5% tollerance resistors are $.05 cents or less there isn't much point in not changing them. In the case of the ones found in Philco 60s many are body-end-dot code carbon resitors of the dogbone style and those are notorious for drifting upwards in value as much as 100% or more, even without being overloaded or damaged in some way. Whenever I restore a set from this era I always cound on replacing every carbon resistor as well as every capacitor in the set, it just doesn't make sense not to.
Regards
Arran