In general, how worried you should be depends on how much money you've made. If you were early enough at Google, LinkedIn, or Netflix, you're in the "filthy rich" category, and you really don't care when you sell or how it happens because unless the company you work for is a WebVan, you have way more money than you can spend. (Note that some times it's very difficult to tell, since Ariba was a viable company but that didn't stop its employees from making some very bad decisions --- An Engineer's Guide to Silicon Valley Startups actually had a few case studies of phenomenally poor decision making)
However, if you're on the edge of serious money (e.g., enough money to never have to work again --- which is far less money than most people assume), then figuring out when to sell is really important. You might think that as an insider for however many years, you ought to have better insight than the market. To some extent, you could be right: when I saw that Mpath/HearMe went public, I knew that the company was not profitable. But that didn't stop the dot com bubble from inflating the stock to well past $50/share before crashing. By the way, one reason to not aim for the moon as far as net-worth is concern is that as John T Reed points out in his book Succeeding, doing so distorts your risk-reward perspective and causes you to take un-needed risk, whereas a more prudent and safer strategy could have netted you more money sooner. Yes, I learnt that the hard way during the dot-com bubble.
I knew that Google was extremely profitable, but because its IPO auction process pissed-off Wall Street, the IPO started out vastly under-valued. Yet, when it came time to really sell Google stock, I found it to be one of the hardest decisions I'd ever made in my life. (One interesting thing about writing that blog post was that many Google employees mis-understood me and thought that I didn't sell any at $700 --- but I was happy to let them think that)
Wealthfront today has launched an IPO sale simulator. I was provided a pre-release version of it and played with it quite a bit, and it does a great job showing you all the different what-ifs scenarios and implications of different strategies for different companies. To simplify things, Wealthfront only has 4 different strategies: sell 10% each quarter for 10 quarters, sell 10% of remaining shares each quarter, sell 50% up front and 10% there after, and sell all after the lockup. My own strategy, which is: "sell something every year but let the price determine how much you sell" obviously can't be represented in any of the mechanical strategies.
One of the things the wealthfront blogpost covers is that if you wait for the second month of a quarter to sell, there's quite a bit less volatility. I think that doesn't quite work: of my 3 IPOs, Google and Pure had very tight trading windows which prevent employees from utilizing such strategies without special effort. In particular, it was possible for me to make far more money from Google stock when not being an employee because I was now free to trade!
Finally, the toughest part about post-IPO selling is that you don't know apriori whether your stock's going to do what Google did, or what DivX or Netflix did. And yes, I do own pre-IPO stock in Facebook, and I have a selling strategy in mind, but I'm not telling, at least not on this blog.