I would say that, in most cases, "pleasant" is prefferable to "pleasing." Any time when you could say, for example, "pleasing news," (meaning news that pleased you), it would be more idiomatic to simply say "good news." Off the top of my head, I can't think of any cases where you would say "pleasing smile" over "pleasant smile," though it's entirely possible that they exist. I wouldn't say that using "pleasing" in these cases would be wrong, exactly. It's just not something that you hear a lot.
I don`t know what the official grammer rule is, but pleasing and pleasant have only a slight difference. I would use pleasant the majority of the time.
You have a very pleasing smile sounds awkward. Whereas, you have a very pleasant smile sounds natural.
In this context, pleasing smile means that the smile pleases me personally. This would be an odd thing to say to someone, unless the context was appropriate. Whereas, pleasant smile just means the smile is nice or pretty. You could use pleasant as a compliment.