I don't see a reason to line a freshly painted cabinet with paper/other, just make sure the paint is 110% dry before you start loading heavy soup cans, and stacks of dishes, your booze bottles, etc., otherwise they will stick and paint will begin to come off when you pull a heavy item from your shelf.

I always wondered about shelf liner too. Anyone else know why you'd do it over a clean surface?

I use non-adhesive rubber shelf liners in the drawers and on shelves. They are washable and last forever.

I know many people use shelf liners, but I guess my point is why would I need to - is there any reason besides personal preference?

I never used them until recently. It's easier to keep things clean (in case food spills in a pantry cabinet). I use the nonslip variety with dishes and cups, just personal preference. I have seen shelves ruined by oil spills, jam explosions, etc. not mine, but I've seen it (I own several rentals, so I see a lot)

if you used latex paint over enamel then you should use a liner?