It's a tough question to answer with a broad brush. It depends on many factors IMO. Here's a couple of thoughts:
Manufacturer: CPC chips need oil, me thinks. They come pretty chalky and they really, really pop with some oil. I like to oil edges and faces since if only the edges are oiled, it might seep into the faces and create a oiled/non-oiled border. BCC, it depends on the chips. The chalkier the chips the more likely I will be to oil them. Some of them do not need it though. Paulson: Same as bcc, it depends on how chalky they are. And in this case, which is the base color. Some colors benefit more than others from oiling. Another benefit of oiling is that it diminishes the color transfer of some chips in my experience. TRK, as DD showed, also benefit from some oiling, depending on their condition. Hardly an heresy me thinks. It's all about dressing up the chips as best as one can. China Clay, not really suited for oiling imo, although there's reports of good results. I tried and didn't like the results personally. Ceramics and BJs, why bother.
For used chips after cleaning, all chips need to be fed oil, imo. Regardless of the method of cleaning, chips loose a bit of they pigmentation and vibrancy (they go down the drain with the hooker juice). They need to be hydrated (with oil and not water as the term suggests), both edges and faces. Some of them might have to be oiled more than once, after a few months+.
Now, in all the oil applications, the amount of oil used should be minimal!! Just enough to make the color pop without making the chips oily and sticky. The action of rubbing them with a cloth is as important as the oil itself (for new chips), so not only one is applying a (very) thin layer of oil but also removing clay dust that might transfer to other chips.
