People don't always like it, but bad form or not, it's extremely important for you to know your relative stack size against other players. Don't make a fuss if they won't tell you, but ask. Keeping track is up to you though, I don't think you should ask repeatedly.
Some players who are not as math aware I think take the question as a challenge - like you're gunning for them. Math based players wouldn't have an issue telling you (although they wouldn't and shouldn't during a hand).