My question is that sometimes if I am unsure of a person's specific title, is it rude to use Mr. or Ms. if they do indeed have a doctorate? I often try to just address them as Dear John Doe, until I know what their title is. However, does it come across as rude or too relaxed if I use both their first and last name instead of a title?
However, does it come across as rude or too relaxed if I use both their first and last name instead of a title?
Depends, but generally speaking there should be no problem with that.
If you have met this person before, or you know them already, using their first names is OK, otherwise I'd try not to use just their first names as it is more personal phrasing.
Using Mr./Ms. + last name is OK to do in case you don't know the specific title they have.
If you then happen to find their title, you can use it instead. However, have in mind that sometimes people don't like being called by titles, so the best course of action would be to ask them how do they prefer to be addressed. Some people I've met even foresee this and ask in advance to drop the titles.
Bottom line, using Mr./Ms. is Ok when in doubt, and use titles only if the person is not uncomfortable or against such use.
The answer is totally country-dependent. I would not advise you to do this in Austria - there you should always try to find out the titles of the person and use them.
More generally, you should always use the norms of the country these persons are in. Try to google these norms - if in doubt, ask someone in your area or ask a question here with country-tag.