It all depends at what your goal is - if you need to buy something with BTC and are going to spend the coins right away, the price doesn't really matter too much as long as you buy it and spend it right away.

The worst time to make any investment is when there is a lot of fluctuation (up or down) as you will never "time" it right ie buy at the lowest point and sell at the highest.

If you are a long term investor then what you should do is "dollar cost average" that is say you have $1000 to invest you make equal investments over time so you are catching the ups and downs, say buy $100 worth every other day.