Depends on how you time it and what you think the prospects are. Skittish Facebook employees missed out on a quadrupling of value if they sold in the first six months.
I might be more willing to accept a guaranteed £X than a 20% chance of a £10 * X payout, depending on the X. My personal investment philosophy generally would recommend pulling out large single investments into broader funds even though the potential top payout drops.
tldr; You don't hold something because you think it will go up. You open long when it is going up and you think it will continue. The reason for the 90/90/90 statistic is because of mindsets like those facebook employees you mention.