From the article that Rick provided.
"There is mixed advice about how to hold your SPL meter to take measurements. Some will tell you to aim the SPL meter's microphone at the speaker you're measuring. If you consistently do it like this for every speaker, you'll be fine. Others will tell you to hold the meter straight up for every channel. Most enthusiasts feel that holding the meter straight up provides more consistent results between channels. (Think about it. Your ears are not directly aimed at your speakers when you listen to them)."
