I am 100% certain that the cards in your packs are decided at the moment of opening. I know this because back when they changed the Priest set, I had 10-20 classic packs saved up, and was immediately able to pull new priest cards.
Obviously they could change this, but hopefully they would announce it first.
As to the OP's question, if you have the ability to control yourself (not a joke, some people are so OCD, they literally can't hold onto packs for any time at all), it's always best to hold packs unless there's something specific you need from them. As long as you wait, the potential to take advantage of change is there. If suddenly you need dust for something else, you can always crack them then.
I also have the class packs from mage and hunter, which I guess would be a good idea to safe?
If you already have all (or all you want) standard rares for mage\hunter then saving those packs is a good idea. You can crack them open once the new mini-set will go live reducing the number of packs you'll need to buy.
As far as I know the contents of your pack are decided when you get the pack, not when you open it. This means that saving them for later has no effect.
This is incorrect. The contents of the pack are generated when the pack is being opened, not when the pack is obtained. You can use this to your advantage exceptionally well with class specific packs, such as the mage and hunter packs we've already obtained through the Book of Heroes and save them for when a new expansion is released. I did exactly this with my hunter pack (I didn't think about it in time for the mage pack) and managed to get 5 Darkmoon Faire hunter cards upon opening it. Just keep in mind, the expansion MUST BE FULLY RELEASED to get the new cards from these class packs. If you use the Fireside Gathering method of early opening on pre-order packs, DO NOT open a class pack as it will not contain expansion cards at that time.
In fact, I'm currently saving my Darkmoon Faire packs from the rewards track and arena runs and opting to not open anymore until the mid-way expansion increase, when the (I believe) 35 new cards will be added to the current Darkmoon Faire set. This way, I'll be able to more easily complete my collection of the new common and rare cards that will inevitably be included as well as give myself a chance at getting some other new cards that are to be released. In other words, I'm guaranteed to get new cards by holding on to these packs as opposed to getting a ton of common and rare duplicates of cards I already have.