Hi John,
Tags, and project tags and Project Types for portfolio projects, don’t really have a direct impact on SEO, e.g. they won’t determine under which key words search engines index your site. Search engines get those key words from your post/page titles, permalink slugs, and the content itself.
The different taxonomies exist for the internal organisation of your site, as a way to structure your content, and that can have an indirect SEO benefit. But more importantly, using those, and using them well, makes your site easier to navigate and for visitors to find content. If you use the built-in WordPress search, or the new Jetpack Search feature, those also rely on categories and tags to find content relevant to the words people search for.
So in that regard I’d say it’s worth using them. Just be careful of duplication – if you already have a project tag for a particular term, don’t use a regular tag for the same term.
In her comment above Liz mentioned the WordPress.com Reader – I just want to clarify that that only applies to posts, not portfolio projects or pages, and also only Jetpack-connected sites with a paid plan are eligible for inclusion in tag listings in the Reader. However, tags do have an effect on how your posts are indexed in the normal Reader Search.
This is not theme-specific, by the way ??