The main reason a rolled piece with a weld gets "pointed" is the difference in tensile strength. Usually, the "pointedness" comes from a weaker spot, such as the heat affected zone. This is especially a problem when bending aluminum. If the weld is stronger, and the heat affected zone is not weaker than the base metal, you will get a flat spot instead of a "point". This may not be as much of a problem on a large ring like 5' dia, but I don't know your application. I think you best bet may be to try one and see if the difference in strength is enough in your particular case. If it does become pointed, or more likely, have an unacceptable flat spot what I would do is roll 2 pieces independantly (half circles) and then weld them together afterwards. You only need a 7.8 ft out of the 10' or 12' pieces you must be buying, so there should be plenty of room to cut off the flat on each end. The other option may be to solution anneal these parts after welding if it is that important to weld before bending, but it may be hard to find a furnace that big that goes up to the temperature you need. My gut feeling is that with such a large radius, and a thin material (I am assuming you are bending the bar the "easy way"), you are not straining the material that much and will not have a problem if the weld is ground properly before bending. (Grinding too much will lead to a weak (thin) spot that will become "pointed too)

Good luck
G Roberts