It is not quite as simple.
A tone control is wired as a variable resistor which controls how much resistance is between the tone cap and ground. A 500k pot gives you more resistance to ground than a 250k pot but if you turn down the 500k pot a nudge, you have your 250k pot. So if you want to most available tonal change on both ends of the control, stay with 500k. But if you want the most even spread of the tone control throughout its travel, go with 250k or 300k.
On the other hand, a volume control is wired as a voltage divider, not a variable resistance. A higher pot has more resistance to ground when turned up, therefore giving a lesser load to the pickup signal. If you turn down the pot, however, you will have more series resistance in line with the amp's input which could then dull the signal. This is way more apparent with an 1 Meg than say a 250k. I think 500k is a value that has proven to work well with humbuckers, providing an acceptable medium between a light load to the pickup signal and not too much series resistance when turned down. If you want to change to 1 Meg I recommend to use a "volume kit" which is a capacitor in parallel with a resistor which are soldered between the input and the wiper of the volume control. This helps to retain the highs when turning down the volume control.
Hope that helps,
Stephan
