unless the odd values are labeled 'for guitar' I doubt that kind of pots would be suitable for us, since they are made for particular purpose, I don't think for example the stiffness of the pot tree is usable for guitar.
Anyway, I also doubt you could actually hear real differences, remember that in electronics there are declared tolerances, on pots these tolerances are particularly wide, in the order of 10% or more, so a 250K could be at least anywhere from 225 to 275 overlapping a 300K pot tolerance....
in case you want to try a particular different value, use a resistor across the outer lugs of a higher value pot, as I just wrote in another post here, the equivalent R is R[SUB]eq[/SUB] = (R[SUB]1[/SUB]R[SUB]2[/SUB])/(R[SUB]1[/SUB] + R[SUB]2[/SUB])