Just go with a complete scan. I don't think anyone ever only scans particular categories.
I never scan all categories. I don't need it, for any reason whatsoever, to know what the price of 97636822 weapons and armors are when my intention is to sell herbs and ores. What I am looking for, in addition, is the option to scan for one product at a time as well, to minimize tedium and time even further. If I have herbed Terocone and Felweed all day, I'm not interested in knowing what previous amounts of weapons and armors are worth, nor do I care about the current prices of Dreaming Glory unless I decide that I know "What's expensive today", and scan the herbs :P
Sorry if I come across as aggressive and demeaning, not my intention, but your statement is just plain wrong :P