I think the decision and strategy is up to the owner. But for me the best time to buy coins is when the current exchange rate is low like when they are on ICO or crowdfunding? If i am not mistaken because i am not so familiar with these things hehe. I once experienced investing for some companies that have great potential after ending their ICO. But unfortunately i sent my purchase to wrong address and I do not have any funds now haha. So if you want to buy the best time is the ICO and the best time to sell is when the value of coin soars high :-)