Wearers of spectacles or contact lenses have a lens between their eyes and whatever they are focussing on nearly all the time during their waking day. It doesn't cause eyestrain and I've always been assured by optometrists and maybe once an ophthalmologist that wearing lenses for myopia doesn't worsen myopia: iirc it's focal distance that matters, and apparently the Rift keeps the focal distance at infinity. Of course Oculus is talking its book, but I'd be surprised if they have no-one who understands the ophthalmology of the lenses-and-screen configuration they're building into their product.