UML is usually a bad smell rather than a best practice.
I've seen good sequence diagrams and good high level architecture diagrams, but nobody decent uses UML. It's far too cumbersome and at the wrong level for an explanatory diagram.
In a bit more detail, and perhaps with less snark:
UML has some decent diagrams. What they communicate, they communicate well. But don't diagram everything - that's a waste. When you need a diagram for something, a UML-ish diagram is fine. When you don't need it, don't draw it "just because". We're actually not in the business of producing UML diagrams.
