On the whole? I'd say we shouldn't.
Edits to questions should respect authorial intent, when possible. It's the golden rule of editing someone else's work: You may have typed the words or made the changes yourself, but you're doing it on the author's behalf. Yes, there are exceptions, and at the end of the day, posts belong to the community, not just the person who wrote them. But eliminating or discouraging (or allowing!) a large number of possible answers that the OP does not want, without any apparent benefit, should not happen. By adding or removing such a tag - really, if it's a meta tag - someone's doing just that.
Incorrect meta tagging has been discussed time and time again on Worldbuilding meta, and the consensus does seem to be that if the question clearly incorrectly uses a meta tag, it should be removed. However, we haven't talked quite as much about when such tags should be added; I don't think there really is such a consensus. However, if we go back to the golden rule of editing, then it's clear that we really shouldn't be adding the tags if the author doesn't want them. When it doubt, ask in a comment, in case the OP simply wasn't aware of the tag's existence. Dialogue with the author never hurts. If they decide that they do want the tag, encourage them to add the tag themselves. Especially if they're a new user, this is a small step towards increasing literacy in the tagging system.
In this case, there's an interesting issue. On the one hand, answers to the question almost certainly need to be based in science. Answers that don't are highly likely to be deleted as Not An Answer, because they just wouldn't address the scientific concerns of the question. From a certain perspective you could argue that answerers wouldn't need the tag; it should be very clear what sort of answers the OP is looking for (and, as Michael pointed out, science is our default). On the other hand, one could argue that the science-based tag is appropriate for classification, which is one thing tags are very useful for.
I'll add a third distinct opinion to the two already here and say that in this specific case, asking the OP is probably the way to go. If they don't want the tag to be used, I'd say respect their opinion. If they acquiesce, tag away. But on the whole, I'll advocate a policy of commenting first, edit later. That's the best way to avoid an edit war.