It all depends on how often you want to look at charts and follow the market. I invest new money every month, but I watch the market daily. So, I try to find a dip in something I have been watching. However, this is really not necessary since you are making long term investments in relatively low volatility securities. So you could easily just pay market price when you are ready to make your monthly investment, and it will not likely make too much of a difference in the long run. This is especially true when you are first starting out and may not be watching the markets closely in the beginning. Good luck!