past or present tense in quoting?

Hello,

Thanks in advance!

What is the convention in using the present or past tense to quote a source? If the present tense must be used, then please explain the reason behind such a recommendation. By the way, I was told to use the present tense a long way back and did not have a sense to ask why.

1. From a written source:
Dr. Samuel writes / wrote, "The medical community ...." I don't understand how he reaches / reached his conclusion!

2. From a recorded source:
President John F. Kennedy says / said, "Ask not ....."

Best Regards,
Hoa Thai