There aren't any concrete rules about tags, but here are some bullet points off the top of my head:
It might actually be a good idea to make a wiki page about tags.
- A tag is not required. In this case you can generally use the sheet as you want, although it is polite to credit the submitter.
- A tag can request permission, credit, or both. Some also request that you don't claim the rips as your own, or that the sheet may only be hosted on certain websites.
- When permission is requested, that means you should ask the ripper for permission before using their rips publicly.
- When credit is requested, that means you should credit the ripper for ripping when using their rips publicly. Obviously it is good practice to credit the original game developers for the actual sprites.
- The previous bullet point obviously doesn't count for custom sheets; in this case the submitter is the creator. Regardless of the tag, it is good practice to both ask for permission and give credit when using custom sprites.
- If you are asking for permission but are unable to contact the submitter, give credit and state that you tried asking for permission. If they see you using their sprites in the future and ask for you not to use them, do so.
- ZIP files may contain a credit.txt file; this serves the same purpose as a tag on a sheet and should be treated as such.
It might actually be a good idea to make a wiki page about tags.