You should line up early if you're really, really concerned with it. However, you can either take chances by
-Moving when people leave between bands, especially if they're very different. I plan on being able to move to the 1st or 2nd row during SC - FK, and then FK-Mercenary, since I think there'll be a lot of switching. I wouldn't count on this for saturday.
-Slowly moving through when people give you any leeway.
There really isn't a bad spot in the house though. Personally, I want a gold badge, and plan to buy one this year if somebody gives it up. I had my taste of the front last year, and it was wonderful. Camera stand, railing, chicken wings, water + headbanging with SwordLord = good.
edit: what annoys me is when I think I have a great spot, the band is about to come on, and then I look behind me and nobody is there. Happens a lot, and especially for Rage. I was in front center for that, thinking "holy crap, I can't believe I'm here", and then halfway through I looked behind me to see 4 guys moshing, and nothing else. Kickass show, that one.