I apologize for the book below. Your answer is that it all depends on the context.
“Was” means that it is no longer true, or it can imply that you doubt what was said or know for sure it’s not true.
The present tense means only that it is true and continues to be so. That’s why it’s the best choice if you know something to be true and ongoing.
So, if you use the past to refer to the present, your audience can believe the action is over or that you doubt it is true. If neither is the case, then, of course, you have confused them. If you know it to be true and ongoing, it’s obviously simpler just to use the present. Less interpretation is involved, and if the audience has no previous knowledge, the clarity is very helpful.
Past = over, possible implication of doubt, or you even KNOW it’s not true
Present = ongoing, no other implications apart from the probability that we believe it to be true
A: Where is John?
B: Betty said John was in Warsaw.
A: Is he no longer there, or do you have reason to think he is no longer there?
B: No, he’s still there. I meant that he still is, even though I said “was.” Sorry.
Do you see my point?
Or
A: Where is John?
B: Betty said John was in Warsaw, but he’s in Munich.
A: How did you know she was wrong?
B: I talked to him on the phone this morning.
(The fact that I knew Betty was wrong helped guide my choice of “was.”)
A: Where was John (last year)?
B: Betty said John was in Warsaw. (This is when it means the past for sure.)
A: So that’s where he was! Thanks for clearing that up.
vs.
A: Where is John?
B: Betty said John is in Warsaw.
A: So that’s where he is! Thanks for clearing that up.
This is easier on both parties. I know it’s not in the past. I have no reason to conclude you doubt it or know that it’s not true. I can take in the information I wanted and ask further questions based on that, or I can leave it alone. I don’t have to figure out what you meant in the first place.
For further clarification, consider these.
A: What does John do?
B: Susan said that he was a doctor (but I think he’s a lawyer). - (Past for present because of doubt)
A: Here’s John now.
B: Hey, John. Susan told me you were a doctor. Aren’t you actually a lawyer? - (Past for present because of doubt)
These are just to hammer the point home:
A: Who is John F. Kennedy?
B: Susan said he was a politician. (Traditional role of the past. We know it’s over.)
A: Oh, did he retire or something?
B: No, he was assassinated. That’s why he’s no longer (or was) a politician.
A: Who is John F. Kennedy?
B: Susan said he was a politician. (Past to refer to the present because of doubt)
A: What? Did he die or something?
B: Oh, I didn’t mean that he ceased to be one when I said “was.” I don’t think he died. I just don’t believe Susan. I think he’s really an artist.
Susan: I overheard you. And, actually, he is dead. So, you were right to say I said he was a politician, even if you used “was” for the wrong reason (ie., doubt instead of it being in the past).
A: Who is John F. Kennedy?
B: Susan said he is a politician.
Susan: No, he’s dead now.
B: Oh, then I guess I should have said she said he was a politician. Sorry. (The traditional difference between the past and present tenses is on full display.)
A: Who is Barack Obama?
B: Susan said he was a politician.
A: Oh, did he retire or something?
B: Well, really, I’m not entirely sure. That’s why I said “was.” I don’t believe he ever was a politician.
(This person is factually wrong, but you can see that doubt can guide the choice of “was.”)
A: Who is Barack Obama?
B: Susan said he is a politician. (There is no doubt that this is and continues to be true.)
A: What office does he hold?
B: He is the president of the United States.
The certainty helps move things along with fewer snags.
Hillary Clinton said she was running for president. I don’t believe her, though.
Hillary Clinton said she was running for president. But then she changed her mind.
Offering an account of the past: Hillary Clinton said she was running for president, and then the race quickly grew acrimonious.
(“Is” is entirely inappropriate for the last two sentences because these actions are clearly in the past. It is not even the present with doubt. It’s just the past.)
Hillary Clinton said she is running for president. (Her first campaign stop is next Wednesday.)
If this were a news story, I assure you that it would begin with “Hillary Clinton said she is running for president” if we knew it to be true and ongoing.