Is it normal to stick your finger in cat's mouth when it yawns?

I don't "yank" my cat's tail, but he likes it when I pet him down his back and grab his tail firmly and pull slowly all the way up until his rear end is off the ground. He also likes really rough massages too. He's getting older and the "tail pulling" straightens out his spine. He really likes that. When I get to the end of the tail and let go he turns around and purrs and rubs on my leg and sits and waits for me to do it again. This will go on until I get bored of it!