One of the biggest parts of cinematic viewing is comfort. No matter how awesome the film is, if you sit in discomfort, disturbance or just right up in front of the screen, it sucks.
This is why I place emphasis on attaining great sits, and stand up against any major disturbance as a result of negligence towards other movie watchers.
So sit in comfort and without disturbance, of course crunching on your favourite snacks is totally fine too!