They say that when all the press are fawning about how wonderful your shareprice is, that is the time to sell.
For example, shortly after the following cover appeared:
Enron went bust, and all the employees lost their pensions (which had sadly been invested in Enron shares).
Apple isn't fraudulent or anything, but it certainly is overvalued. The iphone is no longer the top selling smartphone out there, and since Steve Jobs died, Apple hasn't really come up with any new exciting products.
If you have apple shares, now is the time to take some profits.