Formal letters almost always used to start with "Sehr geehrte Damen und Herren," or "Sehr geehrter Herr Maier" if the person is known. However more and more we see formal correspondence that addresses with "Guten Tag," or "Guten Tag Herr Maier". The former may be less fashionable but is there any consent on what occasions we rather not use the one or the other?
The salutation "Guten Tag" in written communication is a more informal variation of "Sehr geehrte/r....".
Both essentially say the same thing, but "Sehr geehrte/r" has been the accepted way to formally address a person for I don't know how many decades.
In my personal experience, "Guten Tag" has gained traction especially in industries that cultivate a more laid-back attitude - Media, parts of IT, and so on - where "Sehr geehrte/r..." is perceived as overly formal. It is becoming more and more popular among younger people (in their 20s, 30s, 40s) in general, no matter what field they work in. I have seen federal employees in their twenties use "Guten Tag" (although it's not the norm).
If in doubt - say, when applying for a position with a German Bank - you can't go wrong with "Sehr geehrte/r".
For everything else, especially if you are on friendly terms with the recipient, "Guten Tag" (or, if you're really familiar with the person, later, "Hallo") is perfectly acceptable.
In my experience you usually use "Sehr geehrte Damen und Herren" to begin a correspondence, possibly when you're not entirely sure who is going to read/respond to it. As the correspondence continues, it may be adequate to switch to "Guten Tag", as it builds up some familiarity with the correspondant.
It could easily be considered a little too formal to start every single message in the conversation with "sehr geehrte Damen und Herren" if it is already clear who's going to reply to you.
I have never seen a letter (or an email) that starts with "Guten Tag". In what context have you seen this "more and more"?
If a letter is more informal it will start with:
Liebe Frau Müller! (Dear Mrs. Müller)
Liebe Mitreisende! (Dear travel group)
If it is an informal email, you can start with "Hallo" or "Hi".
If you are a native English speaker contacting a German media or communications professional, is it preferable to use sehr geehrte. Guten Tag or Hallo may be used in an informal setting but it may be too informal if the professional in question is not someone I have spoken to, corresponded with or even met before.
If you are an English speaker writing a request for some information about a product or requesting some data, using the "wrong" salutation may not have such negative results as when written by a native speaker. Still, in my media experience, any letter or email that was addressed to myself specifically, or the newsroom in general, that was strangely (read: incorrectly) addressed would generally be a red flag that the subsequent letter/email was going to be garbage.
Is it wrong then to assume that a letter to a German-speaking professional with the wrong salutation will not be ignored or deleted without any response.