I'd rephrase the question thus: To what extent do I want my users to read the site tagline?
If it's that important then you can do things.
Firstly, you want the user to read it once. Banging the user with a single phrase, many, many times, can be annoying.
So you might display it prominently in top pages (home, landing, ...) and make it much smaller in all other pages.
Or, you might redirect a user who comes straight into a deep link through a landing page with the tagline and a "continue" button that takes them to where they wanted to go in the first time.
Or, you can set cookie in the browsers of the users who already saw the tagline, and display it as a fading overlay whenever the cookie is not there (not a flash intro, pleease!).
The idea is that ensuring that the users saw the tagline is one thing, independent of wasting that real estate in each and every page.
What might be more significant is that by posting it again and again you are taking time and energy from your beloved users, both resources you'd prefer that that them used for the activities your site offers.