It seems reasonable to have tags for each security patch, but it doesn't look like they've been used so far. Any objections to tagging patch-specific questions with a tag like supee-6788?
Since the tag supee-6788 has been introduced, there are 70 questions, seraching for "supee-6788" yields 233 results. So individual patches, at least those with bc breaks or other complications could be a topic big enough to warrant its own tag.
But for previous patches there were not that many questions, even for the shoplift patch SUPEE-5344, there are only 63 search results, and for SUPEE-6285 with the infamous "access denied" errors, there are only 61 seach results.
As a conclusion, I would say that an individual tag for each patch is not necessary and there is no real benefit for it.
Remember the reason for tags:
They should make it easy to find questions that you can answer. So for example, if you are an expert for the Magento import, you can follow the import tag (4 followers). Can you be an expert for a specific patch? Maybe, but it seems like that's too specific for anybody to follow: