you can add a logo and keep the site title and tag line via the Customiser > Site Identity. Both will display 🙂
It is really personal preference. For me i like to use an SVG logo that incorporates any text i want in my header logo. I then create a separate stripped down logo for my Sticky Navigation and Mobile Header.
From an SEO perspective the Site Title and tag line will be held in the <title> tag for your home page so it will still get indexed.