When it comes to different philosophies around market timing, first of all, I think ALL investors are market timers to one degree or another.  Secondly, I think it is healthy for the market to have many different points of view - if all investors think exactly the same, then that wouldn't make for a very good market.  Think about it this way - on EVERY single trade, there is a buyer who thinks that the stock will go up and a seller who thinks that the stock will go down.

One money manager who said something that gave ME some pause to consider, is: "If a stock you own climbs to a price that you would no longer be interested in buying it - maybe it's time to think about selling it".  Another adage is that "nobody has ever gone bankrupt by realizing profits" (in reference to selling a stock in which they have a big gain).