#foo { background-image: transparent url(images/bar.png) no-repeat; }
Very good suggestion Bob. Though I do think the "best practices" for CSS really depend on the type of site you’re building. Snippets/plugins/modules definitely make sense to suggest they go in your assets/snippets(or plugins or modules) directory indeed.
For Susan who builds a template showcase with multiple completely unique templates, sticking it all in the assets/templates/template-name/ directory makes sense. For me, I like to save a few bits when working on CSS files that get a bit intense at times.

I have to disagree with any attempt to try and impose structure of any kind on MODx add-ons or external file dependencies, but let me explain a bit...
All good points, but there’s a tradeoff between structure and flexibility and a difference between rules and guidelines.
MODx could, for example, suggest that users put their snippet code "any where they want," (and they could if they wanted to), but having the guidline of putting all snipppet auxilliary resources in the assets/snippets/snippetname directory saves everyone a lot of unnecessary time and aggravation.
Clarification: the main core distributions will not contain an assets directory at all; these external file dependencies will be the responsibility of an add-on transport package to install. But as Ryan suggests, putting together custom distributions of MODx with a specific prescription for file locations will be possible with a MODx SDK distribution, if you still feel the need to impose a set of common standards in your deployments.
Future versions of MODx will likely ship with fewer subdirectories inside of the main assets directory, simply because we don’t believe in prescribing a way to work, and also because we’re going to get rid of the image/file browser that needed so many subdirectories to work in the first place. In fact, future versions of MODx will be entirely configurable as to where your assets reside. We will have the ability to make purpose-specific "builds" of MODx, so a newbie-friendly build with some best practices in place for general sites probably makes sense indeed (including a consensus on "assets/templates/", etc.).

I think what makes the most sense is to mirror how you’d build it in the real world if it were a static site.