Theme functions are handled by the theme registry: rebuilding (or worse, disabling) all your site's caches to pick up a new theme function is unnecessary and overkill.
What you can do is use the Devel module to rebuild your theme's registry on every page reload. Once installed, go to Configuration → Development → Devel settings. There, check "Rebuild the theme registry on every page load" and press the "Save configuration" button. Remember to uncheck it (or better yet, disable Devel entirely) when you're done with development.