When getting stopped like that, I know it's good to have a letter on you from a lawyer explaining why you have so much cash. W2's from casinos help too.

At what monetary amount should I think about getting a letter from a lawyer? 10k+?

So far I haven't bothered to get one. I typically have 5k or less in cash. Is it imprudent of me to not have a letter, or am i correct in assuming I'm not carrying enough to warrant it?