[cratermoon]

This is based on my experience across a variety of startups and small internet-focused companies. YMMV.

Some UML diagrams are useful, but only a small subset of the entirety of UML, and never very formally. Sequence diagrams and class diagrams are especially common. Also, UML officially is very specific about the details of each diagram, but rarely do the details matter very much. The subset supported by PlantUML seems to be plenty. Some of the diagram

Get a copy of Fowler & Scott's UML Distilled and that's as much as you'll ever need to know.