Speaking from my experience as a wedding photographer, I do recommend doing the portraits prior to the ceremony, as this allows my clients to be able to enjoy hanging out with their guests during the cocktail hour, and it also allows them to be able to just relax and spend some quality time together before the nervousness of the ceremony time kicks in.
However, some of my clients prefer not to see each other prior to the ceremony, so in that case I do the portraits after the ceremony. It's their day, and so I'm happy to do it this way if they prefer.
As Courte mentioned, having a romantic "day after" session/TTD session allows the couple to get some great romantic portraits without being tied down to a tight timetable. And, speaking from experience, having a romantic "day after" session is a ton of fun both for my clients and for me. So, it's really a win-win!
Best wishes,
Tod