This is mostly a locational/dialectic difference in English. It is generally accepted that certain areas of the United States and Canada replace "thee" with "thuh" in everyday speech but that speakers from the UK and Ireland tend not to do so. I suspect you'll find that most American speakers in particular do it almost all the time -- except in formal environments, where even the most rural speakers tend to become far more formal and adhere to the stricter rules of pronunciation. Realistically, I doubt most people would find it odd either way as long as you are consistent -- for example, nobody will think it weird if you say "thee ocean" or "thuh ocean" but if you switch sentence to sentence, it will start to attract notice.