My shotty is loaded with 5 buckshot in the tube, and a slug on the end. Nothing chambered. The shell carrier holds 6 slugs,with the primer down. Being left handed, I can simply grab a shell with my right hand and pull it down and quickly slide it into the mag tube. The buttstock holds 6 buckshot, primers up on the left side. This means I have to reach over the buttstock to grab one, but it also means I can maintain coverage on a target at the same time (if I had to reach under the stock, I would need to raise the muzzle end slightly).
As stated, it all depends on how you train. I strongly recommend trying different methods and finding one that is the most comfortable and be sure to load the weapon in he same manner every time.
I suppose the staggered shells do have their uses, but I just can't seem to do it with a tactical reload (meaning, I want the tube filled as quickly as possible).