your truss rod controls the amount of relief or curvature in the neck/fretboard...the instrument is a system and action is a results of what's happening with all components working together, ie bridge, neck, frets, nut...
you can lower your saddles and see how it plays, i'd suggest going in small increments, and trying to turn each screw the same amount in order to keep the relative hts of each string close
if you hold down a string at the 1st fret and then push the string down with your other hand at the 12th fret, you will notice that the string becomes a straightedge with which you can measure the distance between the top of the 7th fret and the bottom of the string...if you get a lot of fret buzz in this region of the neck (middle, 7th fret region), you might need to add some relief, which means you loosen your truss rod slightly by turning it counterclockwise in 1/8-1/4 turn increments (give the neck time to adjust in between turns and don't force a truss rod if it's too hard to turn)
if you play with a lighter touch and your fretwork is done well, you might want to take some relief out of your neck by performing the steps in the opposite direction...
in regards to the upper register, your saddle ht will effect the action profoundly, but the neck relief will also play into the game
i always take measurements of the setup before adjusting just in case i want to return to that feel--might be a good idea so you know you can get it back to how it is now after you adjust things
if you want to learn all the details, buy one of dan erlwine's books, they're great resources
