Depends on the flashgun you have
^
I used the 18-70 /only/ for the last wedding I shot, it's very decent and the dof/bokeh is nice, didn't have the 55mm prime available that day so onmly had that to use.
It's useful since there will be a lot of group shots especially when you are taking one shot and randomly the rest of the friends and family jump in you'll be thankful that 50mm wasn't all you had!
Then again the 50mm will be
best for candids, it will give a much nicer impact on solo shots and headshots of 3-4 people or general walk around grab shots!