So my bass teacher keeps telling me,

"You should head to guitar center to have them set your bass neck if you want to lower your action",

"You should head to guitar center to have them set your bass neck since you just changed strings" and so on..

Is setting the neck really a big deal? and how come none of the guitarists I talk to ever get their necks set?

It is probably twisted, or not totally "right". Maybe your strings makes it goes that way. One sure thing: it's not a good news.
Quote by Slicendice

stop using UG as a search engine.

I guarantee that google will be 10x more helpfull and loving.
im sure he just means 'make minor adjustments to the truss rod and bridge's action'...
if you get heavier gauge strings, there will be more pull on the neck, so the action will be a little bit higher...
on my guitars i usually dont notice, because on guitars its a pretty small difference when you go one gauge up or down.
but if you go say from a string gauge of .9 to .12, you'll wanna get it adjusted
Or, neck angle. A lot of guitarists and basses add shims to change the angle of their neck. Fender MIA guitars actually have tilt features built into the necks of the guitars. Microtilt, I believe it's called. CBS 3-bolt Fenders had them, which was one of the reasons the necks fit about as well my foot in my pocket.
Quote by Cody_Grey102
I was looking at a used Warwick Vampyre LTD 5'er for about $200. I went home to grab my wallet and came back and some jerk with an epic beard got it already..
It is the truss rod.

Example: I bought bass with standard guage strings, thickest being .100.

I moved up to thickest being .105 an so on, now my neck is bow...a good bit actually. So I have to adjust my truss rod(which you want done by a professional.)

Thick srings will usually just pull your neck in a bit (bowing it.) Which may not be a HUGE problem...but it is one.

Makes my action very high in the middle and very low in he high frets (13+) and at the lows(4-).
your teacher is right, take it in and get it adjusted. the UG Bass FAQ has some useful info on this subject. as does this site. http://www.tunemybass.com/bass_setup/
a pro set-up is worth the time and money.