Default styles are there forever, you can't delete them.
Though, I encourage you to use them before any personal style creation, because
-- the stock styles get internationally supported
eg: EN Text Body is FR Corps de texte
-> This is understadable by whoever opens the document, English speaker or French native.
-- any user opening you docs will be able to have their own style settings which will automagically adapt your docs to their taste.
eg: you setup the Text Body paragraph style to use Liberation Sans font fo your document. If you pass it over to me, where I have the same style with the Liberation Serif font setup, I can read *your* document with *my* settings.
Styles are the pillar or OOo/LibO. Just because of this, I couldn't go back to MSO. It is very important to grasp the styles principles and learn their use. This knowledge brings a tremendous step in ease of typing, document structuring and management.
Designing your own styles is of course possible -- and you'll have to do so whenever the corresponding stock style doesn't exist -- but I strongly insist on the use of the stock styles before any other attempt.
HTH,