My first crew I leveled to 80 consisted of Pally/Mage/Hunter/Lock/Priest. All of them were in T5 equivalent full PvE epic gear at 70. With exception of a few pieces here and there from group quests or greens from quests that really should have been blue, nearly all of my gear got me to 80 no problem and I didn't start investing any energy changing their gear out for raid prep until then. I did zero dungeons (and still have done barely any as I'm too busy in Naxx-10/25, OS-10/25, VoA 10/25 and EoE-10 instead). In some isolated cases, I'm even choosing to keep the same level 70 epic equipment I had until I get a drop in a raid because the effort and what I gain trying to get a blue drop or get enough rep to buy a replacement just isn't worthwhile.

Now on my second crew, (DK/Rogue/Warrior/Shaman/Druid) who were also T4 or T5 full epic at 70, I changed things a little bit based upon my experience with my first crew. That is, I skipped Zul'Drak (except for the arenas) and Scholazar Basin in it's entirety so that I would be working on Storm Peaks and Icecrown quests from levels 77 to 80. This is because when I started chasing gear from quest rewards to put on my PvE Raid set, I found myself doing a lot of chains in these zones post 80 with my first crew to get to group quests gear (and also unlock Ebon Hold and Sons of Hodir). I figured, might as well kill 2 birds with 1 stone. At this time I'm still in the middle of Ice Crown and except for my DK, everyone is 80. As soon as he dings 80, I'll be turning to gathering materials to craft my own stuff, since I have at least 1 toon with every professions that is at least 430.

So to sum it up, my suggestion is if you have lvl 70 epic gear, it'll get you to 80 and you don't really even need to look much at the gear you get. Don't trash anything (especially blue gear) so that when you hit 80 and you research your gear changes you don't realize you inadvertently sold something you want to use, but if you're doing multiboxing, probability is your lvl 70 gear will get you to the point where you'll care about gear again at 80 with very little pain.