This link targets tech stocks only. Though the same could be said for any stock. Most that I have found experience a dip, but not all. Facebook (FB) for instance rose in PPS during the "Lock-Up" period.
I'm not concerned about the dips during this period mostly. In my opinion I like the market to decide what the PPS should be for any new IPO. Usually by this time ("Lock-Up" period expiration) the PPS has found a stable price that the market believes it should be. Sure, most IPOs in the past year have done well, but I look at IPOs as a gamble. You really have to put your due diligence to work if you get in on the IPO.