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.
Com'on man, who says it doesn't matter, it really does matters, those that bought and accumulate a huge chunk of Bitcoin earlier when it was way more cheaper, are you going to compare them with you that is just starting? No.
In Bitcoin investment, the earlier you buy the better, that's why waiting for the dip or wasting your time by f**king around with shit coin is a complete wrong an aimless approach when investing in Bitcoin or thinking of investing in it, so just act and start accumulating it once your discretionary income is available, their is no time to waste because time is very important.
As for the part of selling, you don't think about that when you are still in your accumulation journey, it's when you have gotten to the end of your Bitcoin accumulation journey or over accumulation status that you will start thinking of taking profit, not now that you haven't gotten anywhere in your accumulation.