I, personally, am following Suvega's guide and hitting both starting zones.

I am 79, exalted with Valiance Expedition, and on-track with all of the level requirements in the guide. I hit Howling Fjord first and then Borean, but you can swap and still do fine. I'm recording everything with TourGuide so once I ding 80 I'll throw it on our website.

I was level 62 when Wrath came out, and IMMEDIATELY on hitting 68 went straight to Northrend. Most of my characters were boosted so I had less than 200g spread across all of them. At 77, I bought normal flying mounts (self-funded) and cold-weather flying for Northrend and had over 1k gold per character to spare (which I'm saving for epic flying).

I was about 25% into 77 on my slowest character when I started doing Sholazar Basin. Suvega only did 1 starting zone and said he had some trouble making sure he was 67 for that zone since there are prerequisite levels for some quests.

If you're 70, you can do 1 single starting zone and still be on-track with all the level requirements in the guide. However, if you care about your rep I'd suggest doing both starting zones anyway -- you'll hit 74 by the end of the two starting zones and it'll be much easier to fend off PvP encounters in future zones if you're at the higher range.