I'd say: use your own judgement depending on context and on a case-by-case basis.
Each of the books and the TV series contains material that's not in the other, so it's perfectly possible for a question to be valid for one but not the other (it is of course also possible for a question to be valid for both).
Some people may be following the TV series a season or some episodes behind others: depending on when it's shown in their country, or on the DVD releases.
Some people may be reading the books but be behind the most recent publication: they may be relative newcomers to them.
Ultimately if someone doesn't want to be spoilered it's their own responsibility to avoid sources of information that could do so. Using the tagging system as a means of avoiding spoilers seems to twist it's intended purpose.