If you are buying a mutual fund, buy before the closing bell to lock in the price at the end of the day. If you wait until after closing to buy, you'll get the price at the end of the next business day. If the market rebounds, you missed an opportunity to get more shares.
Yes, this is a sort of market timing, but you do have the knowledge that you are getting a better price today than you would have yesterday. Nobody knows what tomorrow will bring, of course.
If you are buying individual stocks or ETF's, you get the price (minus bid/ask spread) at the time that you buy, so it matters less.