Maybe I am just the odd one out, but I would not agree to become engaged unless I actually was ready to get married. We didn't view it as a time to think about getting married; we took that time to plan the wedding!
But we talked about things before getting engaged so we knew we were both ready. In fact, we went ring shopping together although he bought it later on his own.
We did live together for 3 years prior to getting engaged, but we met very young. Granted, though; it's a lot easier to recover from a broken engagement than a broken marriage.
Me: 31 year old poly bisexual Dominant female, married to mono H (married 9 years).