Quick answer, longer control arms work in conjunction with longer springs to locate the axle in the correct position after lifting it with the lift springs. Coil spacers are used either as a boost to the lift that longer springs give you, or provide all of the lift in smaller lifts. If you want a taller lift, research it first. 6" is plenty of lift for most people. Combined with fender cutting, you should easily fit 35" tires. Why do you want more lift?
I doubt your lift kit utilizes the stock lower control arms with 6". In a word, that would be 'bad'. If you already have all of the other modifications necessary(*) to run 35's on 6", then adding an inch or two with coil spring spacers might not be a bad idea. Look into a long arm kit if you are into that type of thing, or drop brackets. Adjustable lower and upper control arms are not a bad idea at high lifts like yours.
*longer shocks, extended brake lines, slip yoke eliminator, upgrades axles, beefier steering, better brakes, or rear discs, etc...