This is one of the oldest pieces of investment advice on record. It sounds so basic that one could hardly argue with it. The answer is, probably not. Following this advice keeps you in the wrong stocks most of the time. The reason is quite simple.
Good stocks are almost never really cheap compared to the other stocks at any given time.
Of course, in bear markets, most stocks are bargains, but good stocks will be relatively more expensive. You generally have to pay more for quality. If you look for price only, you will inevitably wind up with less quality. This actually applies to almost everything. Most sophisticated investors are also aware of the fact that if you bought every new high and sold it three months later, you would make a lot more money than if you did the opposite, buying every new low and selling it three month later.