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I'm still new to kegging but I use it every time I clean and prep a keg for refilling. Not just as part of routine maintenance, but to ensure a good, leak-proof seal. Between tear-downs I just wipe the post o-rings clean with a paper towel and starsan then smear a bit of fresh lube on, wiping away any excess. Cheap insurance against losing an entire keg of beer or a whole tank of CO2. Besides conditioning the rubber material, it also reduces friction and allows better contact between the o-ring and its mating surfaces.
For those, I pick up a smidge from my keg lube container in my fingers and work it around the o-rings.
The lid gasket I take off to get the whole thing coated with a thin film of lube, but the post o-rings I leave on...