How do you market your jars? If you have a 12 ounce jar but the wax pour is 9oz do you say a 12oz candle or a 9oz candle?
I do not sell candles myself, but a friend does. She markets by the weight of the wax. People want to know how long it will last, and that is determined by the wax, not the jar.
msalex, I make 8oz and 16oz. I sell them as 8oz jars and 16oz jars. I fill my jars as full as I can. If I were to put only 6oz in my 8oz jar my customers would think they were getting ripped off. If you put 7oz in and 8oz jar it is going to be full enough to leave room for the wick to stand up and still be able to put the lid on.(that's if your using something like a mason jar). As for your question about a 12oz jar with only 9oz of wax, I would definitley make it clear that there is only 9oz in the jar. You don't want people to thing your ripping them off.
Hope this helps :)