All good questions though

Yes every gun smooth or rifled is affected by the crown.

Because I come from a benchrest and fullbore background crowning is meat and potatoes...you wouldn't even consider not doing it.

Realistically speaking.
Any older powderburner should be crowned because of muzzle erosion
New production air rifles will always benefit from a crowning because of the process they use.
If your modding or doing anything to improve performance the crown should be first thing on the list.

A guy by the name of Harry Pope was so convinced that the cause of inaccuracy was the instant that the bullet parted from the muzzle that he developed entirely new protocols.Known today as The Pope Method. He created a means by which he could machine the crown absolutely square to the line of the bore and machined the end of his bullets flat and developed a means by which to load the bullets from the muzzle so that the rifling smears didn't affect the square edge on the base of the bullet. In 1883 he shot a 25 shot record that hasn't been beaten to this day.

Lots of people will have lots of varying views on the subject of crowns. None of these people have 130 year old records to back up their bull$h!t