Is it necessary to feather all images in photoshop? When should feathering be used?

you feather to increase the smoothness of a selection you want to cut out.When you want a sharper edge lower the feathering,when you want a softer less defined edge increase the feathering.I usually put the feathering at 0.2 by default,if I want the edge totally sharp and cut out detail in something go to 0 and if I want a softer blended edge go to 1 and up.For a vignette effect up to 80 or 100...hope that helps a bit

For the best (most convincing) results, feather any object you chop out of a photo as standard, even if it's only by 1. Sharp, pixelated edges are the biggest giveaway that a photo has been manipulated

oh and remember to always feather a chicken before roasting it in the oven ...HAPPY NEW YEAR PSC !