It would certainly be an option, but I find it more than likely that they just use Dean to improve Sam’s sense of balance. Walking along a finger is like walking along a balance beam, only the skin would shift under his feet, increasing the difficulty and helping him increase his skills. A golf ball might be a bit better than a ping pong ball, since it has more heft to it. A ping pong ball might go flying when he tried to get on.

May 15th excerpt:

Walking along Jacob’s finger like a balance beam, Sam stopped when he reached the center of the broad palm. There was more than enough room for Dean to join him, as they’d found when both brothers fell asleep there. It was bigger than Sam’s old bed in Trails West, or Dean’s nest of fabric that continued to wait for them in his old home, if they ever managed to return.