Cash tips do get stolen from tables, and it is always possible that CC tips can be "stolen" from the server one way or another.
And while it's possible for a server to evade taxes, I don't think that's any of my business.
I was a waitress for 12 years, and as a customer I feel more comfortable leaving a cash tip. Since it's often at LEAST 20% of the bill, I usually give it directly to the server when I can.
I fold it up, though, so the server doesn't know how much it is right away.