f150 question

My daily is an 89 f150 4wd. I have some money that I am going to do some preventive maintenance with. I put my truck in the shop about a year ago and they said I needed to change the idle air control valve. What is this? Is this something I need to fix? I just thought about fixing it since I have a little money to spend on it. Thanks in advance

The idle air control valve controls the trucks idle. Typically when they go bad your truck will not idle, especially when it is cold. Have you had any problems with it idling?

it will idle high when I start it but it will go back to normal. If I start the truck with the A/C on the truck almost cuts off. There have been times where the truck did turn off b/c of it. How much should this cost?