As others have stated, you are better off testing with all five or six chambers. Testing with just one chamber might give you a slight increase in accuracy, based on consistency, but unless you intend to carry and use the weapon that way, it doesn't really make sense.
I like to pick several loads that meet the requirements for the guns intended use, then test groups from the bench shooting 5 or 6 shot groups, depending on the gun. After a couple groups with each load you are testing, the most accurate usually stands out. Some times, more load development is needed to achieve a standard you are happy with. That standard can vary quite a bit, depending on intended use. For example, I would much rather have 3 inch groups at 25 yards with a good self defense bullet like a Gold Dot, then 1 inch groups with a FMJ slug when testing for a self defense load.
Larry