That is a very good question OP.
Consider what you will do with the bitcoins, do you keep them or convert them? If you keep them, use the latest average price, you will notice there is just a 10% difference between the 24h and 7d price. If you want to convert them, use an average in days longer than what you need to convert the bitcoins.
Using an average means you will sometimes lose a bit, and sometimes gain a bit, compared to your desired income. Also ask around to see if your suppliers can convert to bitcoin as well. This way even if the market value of bitcoin varies, you are operating in bitcoin, so any value changes are transparent.