As already mentioned, the usual starting point is 500k for humbuckers, 250k for single-coils (that's regardless of the manufacturer, BKP, DiMarzio or whoever).
The higher the pot's value, the brighter the sound - so 250k works with single-coils, which are naturally brighter sounding, but can make humbuckers sound dull. Conversely, 500k with single-coils will probably sound too bright. If you have a particularly dark sounding humbucker guitar, you might even try going up to 1Meg - which will brighten things up still further but can also make the sound a bit harsh and "ice-picky" (IMO).
Usually, the same value pot is used for both volume and tone, but again you can experiment - DiMarzio at one time recommended 500k volume and 250k tone, then they switched to 250k volume and 1Meg tone, I don't know what they say now..... But anyway, the value of the volume control has more effect on this "brightening up" effect than the tone does, let's not get too complicated.
Don't get too hung up on the "500k vs 550k" thing - there's not a massive difference. As Feline said above, the 550K is
"usually like a 500k ohm pot on the better side of it's tolerance (most electrical components vary in value by plus or minus 10%) and buying a 550k ohm is like buying a 500k pot that is on the plus side". In other words, if you buy a normal 500k pot, it might actually be 500k, it might be 550k, it might be 450k.... or anywhere in between. By buying the 550k, you're just guaranteeing you're getting a good-quality pot which will actually be 500k+.
So, start with 500k (or 550k, if you like), see how you get on. You can always experiment again later - and bear in mind changing the pot values will not damage your pickups, guitar or amp in any way (unless you slip with the soldering iron, that is!
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