I always go with the direction of the fur, especially on my smooth haired pigs. Some of mine like being stroked under the chin. Just experiment. If they don't like it, they'll let you know. My Squeaky doesn't like to be scratched for very long. When she's had enough, she'll kick me with her hind leg.
I find they all seem to like to be stroked from head to bum, although if I put my hand too far forward over their heads, they push it up out of the way.
Both of mine like being brushed, so I do that, even though the one with short hair doesn't need it.