The value of guitar pot you choose will depend on the pickups. Although there are not strict rules, there are some general guidelines that are useful to follow.
Single Coil Pickups
A single coil pickup will almost always use 250k pots. These pickups are bright and airy, and therefore require a pot to dial down the top-end.
Humbucker Pickups
500k guitar pots, if not higher, sound great with Humbuckers. Although they do remove the often unwanted “hum” of a single coil pickup, the combination of two coils leads to a heavy bottom end; the 500k pot allows more high-frequencies through, brightening the sound.
P90
These types of pickups are somewhat of an in-between of the two above, and therefore require a pot in the middle ground. A 300k pot is then a balanced option for this type of pickup.
In Short
Single Coil Pickups = 250k
Humbucker Pickups = 500k
An in-between = 300k
The lower the resistance, the more dampening of the high-end frequencies.
Conclusion
There are no hard and fast rule (and rules are meant to broken). You can use any and all combinations of pickups and pots you like; it ultimately comes down to what sounds best to you.
However, it is important to understand the qualities your pickups and guitar pots offer, and the resulting sound each pairing. Experimenting then is the best thing to do.
