Because discounted cards are basically a direct discount off everything you buy it makes nothing but sense to stock up on them when you find them. Stick them on your own iTunes account and any music, movies or apps you buy come at a reduced price. Often the iTunes cards sales only last for a day or two and they’re easy to miss.
I’m just about to use up $100 of iTunes credit that I bought for $80 from Australia post some months ago. In searching for a similar deal recently I came across the Gift Cards on Sale site which tries to provide a one-stop spot for information on where to find discounted gift cards. I’ve subscribed to their feed in my RSS reader so I get notified when anything comes up. The site doesn’t sell gift cards, it just tells you where to find them.
Right now, for example, you can get discounted cards at a range of BP service stations.