The best investment strategy is to buy cheap and sell expensive, but if we look at bitcoin prices at this time of course make us think bitcoin price expensive, but we must believe that the price will be more expensive so the best time for bitcoin investment is now and do not wait anymore.
That's the basic rules in trading and investing, with bitcoin it's very possible because of the price movement.
When there is a dump that is the right time to buy and then sell when there is a pump, it could happen in both bitcoin and altcoins.