I used to always include one, but changed my mind over time. Some people do, some don't, I split the difference. Common lower value items, no. If going EIS, I usually do (in case the item is unpacked/repacked they have a record of what should go to my customer). All items worth more than my personal risk tolerance get a packing slip (if an expensive item ends up in the dead letter office, or possibly misdelivered, having a packing slip inside may help recover it.)
I don't think for items of small value it is worth it for me personally, over the course of many years the cost of all of that toner or ink and paper would be far more than a one in a zillion inexpensive lost item that needs to be (and can be) recovered.
