I have been playing 5 hunters with RAF until level 55. I had a druid waiting at that level that I planned to swap one of the hunters for. I don't have experience of WotLK instances yet (my group is level 71), but I can share my impressions of WoW and BC instance.

Having a dedicated healer is key. Five pets provide quite a lot of tanking ability, but elite mobs, in particulars instance boss, will go through them really easily. They will certainly die too quickly (at level) to be kept alive by Mend Pet only.

Good tanking pets are also important. I like cats for outside world, but I go in instances only with gorillas. The AOE tanking ability is very precious and lets my druid heal freely, even when adds pop. Priceless.