In my experience this can go either way.
If you buy before the report:
- People might have been underestimating the fundamentals of the company and you make a profit when the price goes up
- People might have overinflated the stock price expecting results which are higher than what is actually released, the stocks go down and you loose your stop loss margin.
- The results are poor and the stock price falls, you lose stop loss margin.
If you buy after the report:
- You may gain better information with which to make a decision about the longer term direction of the company and invest accordingly.
- If the results are good, you have potentially lost part of a long position as the price rises even higher, but done so with lower risk.
- If the results are bad, you can move onto other things.