Joined: Jul 2015
Posts: 7,310
Likes: 3,188
From: NW Oregon
Bikes: 1982 Trek 930R Custom, '91 Diamondback Ascent w/ XT, XTR updates, Fuji Team Pro CF road flyer, Specialized Sirrus Gravel Convert, '09 Comencal Meta 5.5 XC, '02 Marin MBX500, '84 Gitane Criterium bike
i've found that a TINY toe out of the rear of the cage can be helpful in cross chain situations, but is not needed most of the time.... the lower end of the cage sees the small front/small rear interference possibility.
other than that, it's dead on parallel that works best.
the most common mistake i see is aligning to some combination of the shift depressions and the actual cage outer wall... very common on "homeowner repairs".
modern derailleurs have a lot of engineering and testing involved in those cages... like the "farther out" outer wall at the low end of the cage... that portion is not needed for shifts, so extra room is gained for silent running.....
using the CORRECT FD, Designed for the ring sizes/numbers/frame involved, is what's most important.