Normally, once you have calibrated all your speakers/sub correctly to the same dB levels, either by using a Radio Shack SPL meter, or the built in test tones/mic that come with most receivers these days, you normally don't have to constantly adjust the levels of the sub.
Now, some people like to have the sub levels set a little higher than the other speakers, some leave them matched, that is user preference.
I use my receivers pink noise, and set everything to around 75dB's. I have 3 subs spread out throughout the room, and when they are all turned on, they combine to be at around 78-80dB's on the meter from the listening position.
Also, keep in mind room placement for a sub is the hardest thing to deal with to get it right.
