Where is "Kind Regards" or "Best Regards" applicable?
I don't think there is any standard. So I will just voice my own personal opinion.
often used as a standard
Kind regards, Best regards,
...can be used as a standard or slightly preferential
...use for those who are close to you...good friends, family, associates
You can also use as a standard....
Yours truly, Sincerely, Yours sincerely
If it is some that is special to you--very close friends, family, or intimate partners
To be honest, people don't really pay much attention to the way a letter is signed UNLESS they notice that you are treating them in a cooler than normal fashion. For example, if always signed your letter to your girlfriend,
but have recently begun to sign your letters with,
... she might wonder why you have gone from being affectionate to being "cold".
For everyday, normal correspondence, you have lots of options to choose from.
Hope that helps.
Usually I consider "Best Regards" more about the person, than health and well-being. If you want to wish them well for the future, what better way of saying it than using "Best Regards".
People who send their "Warm Regards" are usually close friends, perhaps easier people to get along with or those who like to get along with everyone.
For example, when I talk to people from the UK over time I see this progression as the conversational topics degrade (Cheers, then Best regards, and finally Regards when things get hairy).
There is an implication that a varying amount of "respect" is being paid.
Of course, the rest of the discussion is important, but is it appropriate (or polite) to express this "respect" in a salutation.