I usually tip in cash and write "Cash tip" on the tip line on the CC bill. The following story happend to me 2 times at different chain restaurants and is the reason why I really do not like to tip on my CC/debit card.
I was checking my bank statement, and I had gone out to dinner a few night before and payed with my debit card. I noticed the bill was $10.00 more than I had on my receipt including the tip. I called the restaurant and asked if they put holds on CC/debit card purchases, they said NO, I then explained my charge for the bill was $10.00 more than my receipt with the tip, gave them some numbers off of the thing, and I guess they keep them for a while, so they pulled my ticket. It seems the server was dissatisfied with my 6 dollar tip on a $25.00 bill so added a ONE before the tip and changed the bill total. I guess he thought no one would check that closely.
A similar thing happened at another restaurant as well, the restaurant was a different "chain" but all under the same "parent" company.
I had a friend who worked at a restaurant where they pooled tips, she said that when the tips were large and in cash, the server/bus boy, would sometimes pocket the "overage", and when the tips were on the CC, they were never sure it management was skimming tips because they all just got handed an envelope, no print out or anything for the night. She changed jobs REAL QUICK.
So, I leave cash, LOL.