I determine what arrow length and weight broadhead I want to shoot and then buy the appropriate shaft and tune accordingly. I draw 28 inches and want my arrows to overhang the back of the bow one inch, plus the point.
I shoot mostly wood, which has a wide range of spines so matching spines to the bow at the arrow length I want is not a problem. However, I pretty much do the same thing with carbons. Gone fishing is correct. I buy carbon shafts in the spine that's as close to what I need as possible. Cut them off to 29 inches BOB and then tune them to fit the bow with up front weight and/or strike plate modification as necessary. Almost always, a little tinkering on the front end does the job. This isn't rocket science.
I don't want my arrows longer than necessary because in tight quarters in a tree or blind, for example, an extra inch or two of length can reduce maneuverability. Also, when arrows are in a bow quiver, the nocks can stick in the ground and fill with dirt if one leans the bow against a tree, etc. Depends on bow length, too, of course, but it's more likely to happen with longer shafts. On the target range, arrow length makes no difference, of course, unless you're a gap shooter, where longer length is a benefit.