I ride 32s - currently a Conti F and Maxxis R. I could go a bit smaller for most of my riding, but there is a stone dust rail-trail near my house that I feel a bit better on wider tires for the occasions I ride there.
If you have no specific rough terrain or loose surfaces you struggle to ride with 28s, I would keep them on until they wear out, THEN consider getting slightly wider tires. As someone said above, it's not worth the effort or expense to go up 2mm, but tires are a wear item and will need replacement sooner or later, so for sure try a wider set then.