Originally Posted by
mechBgon
For the DT Swiss rim, standard 12mm nipples are a good choice. The longer nipples would accomodate rims that have the nipple seats recessed further into the rim. The idea behind the various nipple lengths is simply to get the spoke-wrench flats where you can reach them, it doesn't affect the spoke length you should choose.
+1
Since the spoke should reach into and engage the head of the nipple, the length of the (non-structural) shank shouldn't make a difference. There may, however be a minor difference, because the length of thread within nipples varies, and with some - regardless of length - there may be little or no ability to thread beyond the top, reducing your margin ofr error and making a correct spoke length more critical.
Since the spokes I use have 10mm of thread, I use 12mm nipples with 8mm of thread. This will cause the spoke thread to show if it's 2mm to short, or give 2mm room at the top if too long. Total tolerance when I measure spokes is therefore just shy of ±2mm from the top of the nipple. In practice I shoot for 1mm shy of that -0mm, +2mm.
Other than rims with thicker bottoms, the only "benefit" of longer nipples is a longer skirt to hide threads when the spokes are too short. This makes them popular for high production where, for example they might use the same spokes on both sides of rear wheels.