Originally Posted by Steve Katzman
Why would you need to shorten your chain? I also do not follow your logic for the need to adjust the derailleur B-screw.
I tend to be a bit anal about setting up the shifting and my suggestions tend to follow suite. So, you're absolutely right; you don't need to do any of these things unless you experience shifting problems or discover that you have chain slop issues. However, that said, I'll share my logic for "suggesting" that chain length and derailleur angle are worthy of attention when changing cassettes, particularly ones that have vastly different low-gear sprockets.
Chain Length: As for why I (not necessarily you or anyone else) would change the length of my chain, it's because a chain that is long enough for a 32t rear sprocket will be 4.5" longer than it needs to be for 23t sprocket (32 - 23 = 9 x .5" chain pitch = 4.5"). In my book, 4.5" is a lot of excess chain. While it's better to have a chain that's a bit too long vs. a bit too short, the best shifting performance will come from a chain that is as short as possible without being too short. A chain that is the proper length vs. being too long will also tend to minimize chain suck and chain slapping on the stay if you happen to enounter rugged roads every now and again.
Derailleur Angle: Rear derailleurs are designed to work with gearing that falls within certain parameters, i.e., a given capacity and maximum sprocket size. A long cage (SGS) XT derailleur is designed to work best with a 34t max sprocket (off-road gearing) & triple crank set-up whereas the Ultegra/DuraAce/105 long cage rear derailleurs (GS) are designed to work best with a 27t max sprocket (road gearing) & triple cranks. In addition to the length of the cage -- which helps to pick-up the extra chain required for the larger capacity of a triple + 34t sprocket -- the derailleur itself is a bit different; optimized for use with the max size sprocket. You can more easily exceed the capacity of a GS-like RD (short/medium/road triple) by using a rear sprocket that is too big vs. running a SGS-like RD (long/extra-long/off-road) by using a sprocket that is too small; however, shifting is not optimal with either.
The angle adjustment screw, or B-screw as it's often called (Shimano's contribution to RD lingo), is used to position the jockey pulley and, as you would expect, screwing it in moves it away from the cassette and screwing it out moves it closer. You adjust the B-screw to set RD to be as close to the largest sprocket as possible. If the B-screw is too far out, the jockey pulley can end up dragging against the largest sprocket when the bike is in its lowest gears which can create shifting problems. Conversely, if the B-screw is too far in you reduce the angle between the jockey pulley and the sprockets: ideally, you want that angle to be as great as possible for crisp shifting, noting that the derailleur works by pulling the chain up and down the cassette by dragging on or off sprockets. Therefore, any time you change a rear cassette where the largest (low-gear) sprocket size is changed the B-screw "should" be adjusted and, as already noted, the chain "should" also be checked for proper sizing to ensure it doesn't end up being too short or too long.
In short, the rear derailleur / cassette / chain are all part of an integrated system and changes to any one part in the "system" usually require an adjustment to one or both other parts get optimal performance. If you don't mind sub-optimal performance, then adjusting the chain length and/or B-screw adjustment is not necessarily "needed". For me, given the scant amounts of time we have for riding, I place a high value on problem-free performance and do what I can to keep it that way. Clearly, it's not essential for everyone...