[exray]

For example if you need a 50K ohm 1/2 watt can you use a 47.5K to 52.5K 1/2 watt resistor, within +/- 5% ?

47k is the normal replacement value for an older 50k.

I believe I read that caps can vary by 20% of specified value, is this correct? ? If so, does in hold true for all caps electrolytic or non-electrolytic ?

More or less. Old electrolytics sometimes had a wide range, as much as -50 to +100%. That doesn't imply that you should use a replacement that far off but the standard values, just like the resistors, are different these days. Use the closest available value.

What do you do in a case such as " .015 - .015 mfd - double bakelite ?

.015 can be found. This may be a loaded question in this example if that is the AC line bypass cap. They were often a dual .015. There's a special type of cap that is preferred in AC-line bypassing nowadays.

No need to get nutty trying to achieve the exact values of the old components. If you're within 20% you'll almost always be within spec.

-Bill