I've been to WDW literally 1000+ times, and I definitely vote for heading left first. You completely beat the crowd that way, as other posters have mentioned. If you keep to the circuit, and don't criscross the park, you end up in Tomorrowland in the evening, when the crowds have cleared out. Bonus points if you don't care to see Spectromagic, as you will have NO waits or very, very short waits while everyone else stakes out spots for an hour or more for the parade.
The only caveat to this touring plan is if you really, really want to see Spectromagic. Then you may want to start in Fantasyland, then Tomorrowland, and follow the circle from there so that you end up in Frontierland last. That's because you can usually ride Splash and Thunder Mountain with little waiting while everyone's holding their spots for Spectro... then head up to the rope right there in Frontierland at the very end of the parade route. When we were there Friday, we got a front-row spot for Spectro in that area, literally walking up less than 3 minutes before the parade reached us. Sure beats waiting an hour or more for a more popular spot.