Never EVER carry more than 10K cash.
I’ll probably never have to worry about that.
There are plenty of situations these days where even with big ticket items, cash is king. If you're buying a rig or airplane from an individual, even cashier's checks are no longer safe. As others have mentioned here, it is not illegal to carry cash in this country (yet). I know I sure as hell wouldn't declare any cash I had to some thief that works for the government for any reason whatsoever, because they are as likely as not to "confiscate" it from you, especially if it's not enough money to make it worth your while to fight the seizure.
The entire concept of the government just confiscating money from citizens because they feel like it is evil incarnate. If such a thing were to happen to me, I'd probably end up in jail, because I wouldn't be satisfied with 'legal' remedies.