After doing the first Maven
You can ignore styles.scss.cache. It is generated by the Vaadin servlet after it compiles your style for the first time. Cache entry is controlled by VaadinServlet on row 1107 and it will be good to let the server to create and invalidate this file when necessary.
On the fly compilation of SCSS works fine but if that turns out to be "too slow" you can compile your theme to flat styles.css file with maven. For example, Vaadin is compiling Valo theme into vaadin-themes library and you could do similar artefact from your theme. See Vaadin theme documentation for more details.