Worth doing a "purge" also before saving as a template, to remove unused stuff.
I got this wrong when I made my first template - all my first projects had +many MB of file size from the components I made to give the new styles a workout. Doh!
The template is really just a regular SU file, so you can include anything that you find useful; components, layers, styles etc. - even geometry in the workspace if you wanted, say, a grid of guidelines always present.