That is fantastic advice which is true for all investments, Gold, Property, Stocks, you name it. The trouble is you never know what the yearly low was until you are on the other side of it making yearly highs!
BitCoin has several problems which will prevent it from ever becoming a currency, and that it what it was designed for, the word "coin" is in the name, it is not "BitBars" as in a store of value like a bar of Gold. It has now become more akin to a store of value but with huge risk (hence the huge rewards so far for some).
Greyscale buying is a concern, because think what the price would be if they were not buying those quanitites. Sooner or later, they will need to stop buying and sell. Who will they sell to and what will that do to the price?