present or past tense?

hi, pls help; this qn has been bothering me for a long time, and no one seems to be able to give me a satisfactory answer.

After words like thought, knew or said, do you always have to use the past tense?

i.e. i didn't know the earth was round vs i didn't know the earth is round

after all, the earth being round is a fact isnt it? and should facts always be expressed in the present tense?

pls help thanks!