There is no "approved" method - it is up to you
I personally put the edition number (when relevant) as part of the Title for the following reasons
- I want Calibre to treat each edition as a separate book
- I want it to be easy to see visually when looking at a title list that I am interested in the Edition for this book.
If I will only ever have one edition of a book, then I tend to put it into the comments instead.