Yeah, it's hard to tell what Apple is going to do. I sold off a few shares on 7/5/12, at $613 apiece, to rebalance a bit. Apple was getting to be too big of a chunk of my portfolio, so I wanted to trim it a bit.
I've owned Apple since early 2005. Originally bought in at $83.60 per share, but a few weeks later it did a 2:1 split. I bought a few more shares around $37 per share, and some more around $114. In late 2007, Apple briefly went to $200 per share, only to bottom out around $80-90 at the trough of the Great Recession. It was back up to around $200 by early 2010.
Wish I had thought to buy more when it bottomed out, but instead I bought some Nordstroms. It was down to around $10 per share, and it went up even quicker than Apple at the time, hitting around $38-40 by early 2010.
At this point, if Apple drops to around $525-550, I might buy a little more. And, if it goes up to around $650-675, I might sell a bit off. But, nothing too drastic.