The best time for buying is at low price and I think if you sell your coins with profit then you should not regret after Fluctuations in coin price because you have already taken your profit so ready to make another step instead of regret about past.
If you are in doubt about other coin then just buy bitcoin now and don't sell until you achieved your target. This time is not suitable for selling but if someone is in huge profit and he cannot hold longer then selling should be preferred. Understand the market and accept your profit but I think if someone is selling their bitcoin without waiting more so he will miss the opportunity of having huge profit in Bull season.
for me the time you buy doesn't really matters but when you sell, if you have a plan of selling when the value is higher then no matter what you spent during your purchase you are sure of making some profit because you already have a plan and you are committed to this plan not minding what the prevailing circumstances are, and then when it is convenient for you then you carryout your transactions, perhaps this is where the long term approach comes in.