Generally I'd say no if they are being picked up and held like it - it's a very unnatural posture and most likely to remind a bun that it is being picked up and carried off to be eaten. If they flip themselves over onto their back while lying relaxed, that is a different matter and shows they feel content and safe.
Flipping most buns over onto their backs makes them freeze and is called 'trancing' and is generally (although not universally) considered to be extremely stressful to rabbits. Because they freeze, people assume that they are relaxed, but the general theory is that it's an instinctive response to being held by a predator and is therefore very stressful. Personally I wouldn't turn a bun onto its back apart from out of necessity for health checks (and then I would hold the bun not trance him or her), but I would be happy if my buns felt secure enough to throw themselves onto their backs to sleep.