Purely cosmetic, most barrels protrude a bit. My FLG has a regular customer who pays extra for 6" barrels to be cut to 5.15" because he likes more protrusion than standard. Makes them look like Llamas to me. Ick.
Note that a truly flush muzzle would have to be at the 1.5 degree lockup angle of the barrel relative to the slide and not square to the bore.
Another alternative is finding a thicker barrel bushing (EGW's comes to mind). The thicker bushing makes the barrel seem even with (or slightly below) the bushing. I like this look, as well as a nicely "countersunk" crown job.
Actually, in a 5" it is just under 1 degree.
King's bushings are plenty thick enough to machine on
Crowning is done to improve accuracy.
Flush crowning is done to improve accuracy, protect the end of
the barrel and look really cool.
I hadn't thought of facing off barrel and bushing together flat and flush at the lockup angle.
Now you mention it, I bet you could include the plug and FLGR tip, too, the whole front end of the gun in one plane.