Hi chopski!
Welding cylindrically shaped objects is considered more difficult, because you have to alter the angle of the welding torch/welding electrode constantly! The potential for introducing stress into the weldment is also increased because the heat input on the weldment is in a state of Inequilibrium, if you don't think things thru!
Welding from the bottom (6 o'clock) up to the top (12 o'clock), in one go, then doing the same on the other side, will result in uneven stresses on a cylindrically shaped object. However, if you weld from 6 o'clock to 9 o'clock on one side, and 3 o'clock to 12 o'clock on the other side, your reducing the stresses as the stress is opposite each other!
Hope this makes sense!
Regards