There is no inappropriate time. Some couples have the fantastic ability to see a photo of themselves at the moment they first met, or very shortly after, it can be a priceless treasure years later. Some people use a couples photography  to show of to the world that they have met, and are pursuing some form of relationship. Again, at any time. Certainly, at the time of engagement, is a well expected time to have a couple shot.

If it is done professionally, you have the benefit of something that looks very good, to almost everyone. But often an amateur can catch a couple shot that is great, as well. It may, or may not imply a further aspect to having a couples shot made, but in almost all cases, it is something that will be treasured, or maybe laughed at, long, long afterwards.