[morzh]

Most resistors could be measured in-circuit as often times they end up driving a tube's electrode and have no DC path to complete the circuit. If a resistor measures slightly below the declared value it is most likely OK and can be left in place; if it settles above the value outside 10-20% it then has surely drifted by that much. If over 20% it should be replaced, especially if it is a part of a divider where precision matter more than in, say, being a grid stopper.