In general, the “buy low; sell high” mantra is a generally accurate mindset most of the time:
* Don’t buy anything at an all-time high. 
* Don’t buy at a very inflated price because of “Fear of Missing Out”.
* Most coins will dip after having a major surge (be patient).
* If a coin you’re holding has a massive surge, sell/trade some of it to hedge bets.

However, sometimes you don’t want to buy at lows.  If a coin is dropping in value, it may be better to wait to wait for the price to stabilize and the recovery to begin rather than  you may want to wait a while for the price to stabilize and start to recover before picking up.  If a coin is in a descending pattern, I want to let it finish dropping so that I can buy it when it fully corrected.

REQ (relative to market cap) – Jan 2018

If we look at Request Network (REQ), for example (good idea, good team behind it).  I recommended it to people when it was 30 cents, but it shot up to a dollar very quickly and became overvalued.  Those who refused to buy it at $1 were smart.  However, even at 75 cents roughly a week ago, there was no apparent sign of the correction finishing.  We can’t just assume that a coin will finish correcting at arbitrary point.  As you can see with the visual, it’s a clear-as-day descending triangle pattern and we needed to see if it was going to drop back ot its original price or not.  I was patient and waited for either the triangle shape to fully form or for it to break out of its triangle.  Now, it started to break out and it’s a good buy at ~50-55 cents.

It’s true that buying a coin after it’s beginning to surge doesn’t net as much ROI as getting it before the surge.  However, I do this for two reasons.  First, I do this to reduce risk.  Second, I due this to reduce opportunity cost.  Buying a coin at its lowest while it is ranging may offer more ROI, but it takes an additional week or two before the surge actually begins, that’s resources that could’ve been put into better coins.