Valve alignment is checked by eye using a mirror and/or fiberoptics and a measuring tool which lets the technician check the for the proper stroke length for each piston and visually check to see if the ports in the piston match up properly with those in the casing. Manufacturers use jigs to assemble and align the parts of the valves and casings prior to soldering. Although most would like us to believe they are dealing with the great precision associated with the aerospace industry, the fact of the matter is that tolerances are broad. In brass instrument assembly you are looking at .003" to .020". A sheet of normal printer paper is approximately .008"! A valve and port that are .010" or greater out of line can cause significant resistance in a horn that a sensitive experienced player can feel. It is also not uncommon for one port to be right on and others in the same combination do not line up. This is a manufacturing problem associated with the jigs during the port soldering stage and is not correctable. Only recently has this problem been brought to light and I am not aware of any manufacturers that consider this a problem since most players are not aware of its existence. After doing a visual inspection and measuring for the proper stroke the technician can decide what combination of corks or felts are needed. In those expensive valve alignments you read about materials other than cork or felt may be used to eliminate the minute differences in thickness felt resents and also to reduce frequent readjustments due to compression of the felt. In some cases I have seen the top side of the valve stem where the felts and corks rest, have to be machined own in a late because the thinnest felts made were still too thick. If there is a condition of one port on and the others out of line the experienced technician will try various compromise adjustments with the player trying each one to see which works best for that instrument and player. Often notes are made and given to the player to facilitate changing felts or corks later since one valve may be different from another. This will eliminate the need to have another in depth assessment of the alignment already paid for by the player. Rotational valve alignment presents us with other much costlier and complicated problems. Keep in mind this type of adjustment is not for everybody. These problems have plagued brass players since valves were invented. There have been I know world class players who could care less and other world class players who won't let anyone near their custom valve alignment. Undoubtedly the problem exists and the manufacturers must be pressed to increase quality. Hence the proliferation of custom instrument makers who strive to satisfy the need for higher quality instruments. There are many reputable shops (most in major metro areas) who offer this service. Expect to pay $90 and up for this service.