I had a customer complain about the seams on the corners of a fondant ribbon. The ribbon trimmed the bottom of each tier of a three tier square wedding cake. I trimmed the corners to make them flush, then let them go at that. It was a traditional cake, with only frilligee as additional decoration. I have high standards, but I thought it looked good. The bottom tier was a 14 inch square cake. Too big for one fondant strip, I think.
Seamless fondant ribbon?
It's hard to use fondant as a strip around cakes, because as you know it stretches. Working with small pieces is best when it comes to fondant. The best solution to your problem is to use modeling chocolate instead. It doesn't stretch when you work with it and you can wrap strips around cakes and have only one seam.
To see more about modeling chocolate, check out this thread:
I use a pasta rolling machine to roll out my fondant ribbons for cake decoration. I wind them around a dowel rod and then unroll them around the cake to prevent streching. That way I only have one or two seams to smooth together. An easier way to smooth them together is to leave a little extra at the beginning and the end. Match them up to their edges and the corner of the cake. Cut them with scissors flush to the corner, then smooth with a little water on your fingertip to ease the corner smooth.
Hope you can understand my directions, I can show it better than I can write it!