Different string guages will put different amounts of force on the neck. Heavier strings will pull it foreward more. The truss rod is there to counter that force.
Scrubs is entirely correct - don't adjust anything if you're not confident you can do it properly.
When you change string guages, at the very least you should set intonation. If you change strings with the same guage you had on the guitar before, things should stay put.
Sometimes changes in humidity will cause the neck to move around a bit. You might have an axe that plays great with no buzz and ridiculously low action for 6 months only to find that one day it starts to buzz. This is fairly normal.
There are 4 basic conditions your neck could be in:
Dead flat: bad
Back bowed: very bad
Twisted, or rollercoaster: fucked
Bowed foreward: good
When the neck is bowed foreward, it gives you something called "relief". When the relief is set properly, as you fret each note up the string's length and the fretboard, there's a tiny amount of clearance to all the remaining frets towards the bridge. No buzz. It can't happen with a neck that's flat, and certainly not when it's back bowed. The relief should be visible looking straight down the neck from the headstock. There are several ways to check relief.
Adjustments should only be made with the intended string guage installed and properly tuned. If the curvature (relief) of the neck needs to be changed, you adjust the truss rod.
Warning:
If you adjust the truss rod improperly, you could permanently screw up the neck.
Adjustments should be made very gradually, and the neck should never be allowed to back bow. Try 1/2 a turn (maximum) at a time on the rod, then wait a couple of days for the wood to settle down. If you're already in the right ball park, 1/2 a turn could be enough of an adjustment. Too much of an adjustment at once could allow the neck to twist. Very bad. Re-tune, play, set bridge height etc... wait... repeat if necessary.
I've also heard of people detuning their guitars for storage, or storing them with no strings. Very bad idea. The tension from the truss rod will cause the neck to back bow. Given enough time to settle like this, you'll have a hard time bringing the neck back to where it should be. If you're just changing strings or cleaning the neck, or putting teak oil on the fretboard, it's ok as long as you get the strings back on there asap. Changing strings one at a time is a good practice, as the neck doesn't have a chance to move around all that much.
If this is an older guitar, relief might not be the problem. You could have high frets somewhere, or they could be worn. If that's the case, you'll need to take it to a setup guy or luthier to have the frets dressed or replaced. The same person should do a proper setup at that time.
$50.00 for a basic setup isn't that much compared to the value of your axe.
If you want to take a shot at doing a setup, there's some valuable information here:
http://www.fender.com/support/manuals/instr_owners.php
Scroll down to where it says "Current Owners Manuals".
Right click and save the one that says "Fender Electric Guitars and Basses".
There's some decent information on checking these things that should get you into the right ballpark. How you fine tune things is up to you, but this is intended as a guideline for procedures only. Pages 8 and 9 have some pretty cool pictures for setting relief and bridge height. I usually just wing it for my own instruments, but these instructions might help if you don't know how to do it.
Gawd, what a long post. I hope some of it makes sense.
sl
