Your budget may have something to do with your decision too. If only the idler arm is worn you can slap on the new one and go driving. If the car needs the other parts too, you're also going to have the front end realigned.
Idler arm only....$25~$60. Rock Auto is having some closeout prices on name brands which are at the low end of those prices.
Whereas replacing all of the steering parts, plus an alignment, is going to cost you upwards of $300.
And when replacing the idler arm, notice that the mounting holes on the car for it are slotted. You want to slide the idler arm up or down to get the center link parallel to the ground. If you don't, proper to-in/toe-out will be affected while making turns, which will hurt handling and tire wear. Your front end won't need an alignment if changing only the idler arm.

 

Howcome the dealer said that realignment is needed when he replaced the idler arm of my car? Was that really necessary or he just made me think that it needs realignment to pay more for the labor?

 

The one thing that could get thrown off is the steering wheel centering, meaning going down the road, the steering wheel could have been rocked at an angle.
Not only can that be an annoyance to the driver, but it can affect turn signal canceling too if it's far enough out.