When I'm shooting a wedding I don't keep a set time of when I shoot the rings. If I'm mid shoot and I see something along the way that I think would look good for a picture with the wedding rings then I'll shot them right there. If I don't see anything during the formal photo shoot then I'll wait till there is a bit of downtime during the reception and I'll grab the rings there and use something at the venue. Either way is good, but it's a key photo that definitely should not be missed when shooting a wedding.
Thanks for stopping by, Rob