Not sure there is a best practice. This is what I do -
I found that having unknown or no date of birth meant that I would have to look at a number of people with the same name to find a particular one, this got tedious. So I now put in c.1820 or whatever, and a place of birth even if it is as general as Scotland. This gives me context for people taken individually as a list of names to look through even if it is only very approximate. I put in the description field how I have estimated the date of birth, eg. "date of marriage", "age at death", "parents date of marriage", "mother's age".
Sometimes these are +/- a year, sometimes up to twelve years or so, but given that I have many names that are repeated multiple times due to Scottish naming patterns I find it necessary and useful.
I would love it if there were more forms of approximation for dates but circa or about are the only ones that seem to be universal. My main program FTM gives me a few more such as between and before, I do use them but they don't upload correctly to Ancestry.
