Tell you what, there have been so many times when I have initially dismissed something as not possibly having any symbolic significance. Front-Ranger's thread about buckets comes to mind, an observation about the meaning of buckets in various scenes corresponding to Ennis' situation at that point, that as the thread unfolded also entailed the symbolism of coffee pots, which represented Jack's situation, and I think eventually led to animals and maybe fans and ...
Anyway, I remember seeing the bucket thread title on the forum index for days and days and not reading it, thinking, "Yeah right ... buckets?! I think we're getting a little carried away here." And then I finally in a moment of boredome checked it out and ... oh. my. god. Every single example she mentioned worked perfectly as a metaphor for whatever it was in the scene it was in -- all different but connected, all with undeniable significance -- waaaayyyy too much to be accidental. I think my first post on that thread started with the word, "Whoooeeeee." I couldn't believe it. But once I thought about it, it made perfect sense.
That was where I learned my lesson. I never dismiss an anything as just an anything until I really examine it closely. (Or, well, to be perfectly honest, I still do that sometimes -- and still usually realize later, once I look hard enough, that I was wrong.) Now, obviously there's plenty of room for argument. Nobody knows what the filmmakers' intentions were, and we can read all kinds of things all kinds of different ways. And it's even possible that sometimes it really does mean nothing, that something's just there because it's there.
So some people here say characters in the movie spit just because people spit in real life. So OK, as a test, can anyone think of a spitting instance in the movie in which the context of the scene demonstrably does NOT include contempt or dismissal or something pretty close? Anyone remember a time when someone spit when they were happy or cheerful, as they might easily do in real life?
In any case, no need to be evangelistic about this. Symbolism and metaphor is supposed to enhance your appreciation of the movie. If it doesn't work for you, that's perfectly fine ... obviously you enjoy the movie anyway or you wouldn't be here. But I would hesitate to discount what others see in them.