It's tough to make a hard decision on this at the moment. We can't really anticipate yet what the question landscape around here is going to look like and it probably depends a lot on that.
For now I'd suggest using a finer granularity. Separate tags for separate shader stages definitely make sense. We can always create synonyms or burninate tags later on. My general opinion on tagging is that there's a reason a question can have up to 5 tags. As long as we don't run into trouble adding all applicable tags within this limit, I think more is better, because it allows for more effective filtering of questions.