Only medical doctors and full time academics should use "Dr" in their titles.
By full time academics I mean those who have AT LEAST THREE research papers published in RESPECTABLE journals, and who are holding FULL TIME teaching positions in a reputable university or institute of higher learning (post-secondary). Those tutors out there who teach at tuition center who want to use Dr can fuck off. Please be sure you are a university lecturer first before you use Dr.
As for Dr Martha Lee, well, I think her videos are very nice to watch.