Depends on how lurid the cover is (I read Baen; I have a high threshold).
Seriously, never. I'm pleased to be reading, and have the leisure time to read, and am lucky to have neither physical issues nor any other qualms that might make reading paper books difficult.
Mind you, I rarely if ever cart hardcovers around simply because they're an inconvenient size to lug around with the other stuff in my backpack if I'm going out.
Only if the book's very compelling; otherwise it's PB or Kindle for my portable/public transit reading.