If it was my vehicle and I had just replaced an idler arm I would get an alignment. You should take most of what you read here with a grain of salt, but i suggest going to a different shop because that tech telling u ya don't need an alignment after a critical steering part is changed should go hit the books some more. Either way an idler arm is a piece in the vehicles steering system, when it is changed it will change vehicle alignment readings. Most specifically toe, toe is the angle that when incorrect will cause tire wear most rapidly as compared to caster, camber, sai, ia ect. So pay 60-100 more dollars and save your much more expensive tires sounds like a no brainer to me.