From my perspective, it makes sense that this tag request was rejected.
Edits (among other things) are supposed to preserve the intent of the post. As such, software-recommendation alters the scope of the post by deviating from the initial intent of normalizing MP3s. Note the tag description:
Questions asking for specific applications that perform a given task or work with a given file format. Give details about the purpose of the software you want and your minimum requirements for it.
While, yes, software recommendations might be a byproduct of this question, it is not the intent. Therefore, the tag is irrelevant to the question.
However, my own general rule for tagging can be summed up as follows: use as few tags as necessary. Meta.SE has been kind enough to provide their own tagging rules that effectively say use as many tags as possible, but ensure that they're all relevant. In other words, aim for five good tags, but don't stretch it out.
For a better example, look at this question. The usb tag is completely unnecessary and does more or less nothing to properly categorize the post. Therefore, it can safely be removed without making the question any harder to find. USB is relevant in the scope of the question (debatable, but let's assume it is), but it is not an important enough part of the question to warrant it being a tag.