When to manually recompile a updated Design System
Anytime anything is changed in src folder. This means that the following does NOT need to build:
Update to templates
Update to layouts and helpers
Update to fragments
These things DO need to build:
Any work on custom components
Custom CSS or JavaScript
New or updated fonts (unless it's just the files and they have exactly the same file names)
config.json is a bit special in that some things can be changed without needing a build, e.g. colors.
Read more about the process to Make Manual Updates to CSS and JS here.