Volume pot value matters most if you tend to leave your volume up full. The moment you turn your volume down a bit, you're loading your pickups down and pot value starts to become moot.
Consider that your volume pot presents two parallel paths to ground from the volume-pot wiper that the next device in line sees. If the pot is up full, one of those paths is the full value of the pot (i.e., 250k or 500k) and the other is whatever the DC resistance of the pickup/s is (somewhere in the neighbourhood of 6-9k). Lets turn down the volume a little. So, now we have, one one side of the wiper, 200k going to ground, and 50k put in series with the pickup/s. We are now pitting roughly 58k against 200k.
From another angle, consider that, when two resistances are placed in parallel, their effective combined resistance is determined primarily by the lower/lesser of the two. So, if the next device in line would "prefer" to receive signals from something with an output impedance of 10k, putting 250k or 500k in parallel with 6k will still get you there (it will be <6k). If we place 200k in parallel with 56k, we end up with around 44k in parallel, which will end up losing us bandwidth. This is why many guitars use a bypass capacitor on the volume pot; so that high frequencies are treated as if NO resistance are added to the load of the pickups from the amp's perspective.
