Should teachers announce the grade breakdown during first class?
I think it depends on the situation (assuming there is no official department or university policy on the matter).

For example, I am a young (math) teacher teaching a single course that has 25 students in it, at a college that I have never taught at before. I figured it was unlikely that I would be able to write exams that effectively separated the A's from the (A-)'s (say) based on some numerical scale that I set ahead of time. So I chose to not put a grade breakdown on the syllabus.

After each exam, I look at the performance of the students, and I give them an individual grade update containing what I call "a good estimate" of their letter grade thus far in the course. This prevents them from just remaining in the dark all term long with regards to their grade.

I have taught sections of courses at bigger schools where the grade scale is set ahead of time and is the same for all 800 or so students enrolled in the course. This makes sense, as the content of the courses is the same year after year, the exams are similar every year, and in general everything involving the course is somehow standardized.

I would say that, in general, it's fine to not announce a grade scale ahead of time if you don't have to. Just be prepared to have something to say about grades, because students will likely want to know. Sometimes I give myself some wiggle room on the syllabus by saying "your grade will not be lower than the following...", so they know that a 90 (or whatever) will guarantee them an A, but they might also earn an A with a score of less than a 90.