In many instances the two wings were assigned separate picket duty and when the first shots of battle occurred one or the other flanks had to be held by the strongest companies on the field to maintain order.
The same was true in line of battle,but proximity to the colors also caused heavy casualties. For instance at Murfreesboro Co. C. 7th Miss. was almost destroyed because they bore the colors. The same had occurred at Shiloh and Munfordville prior to Murfreesboro. Co. I and Co H also took heavy casualties because of their closeness to the colors to the point that their Captains had to return to Mississippi to recruit replacements.
After Murfreesboro almost all companies had to be consolidated, but to maintain order, the consolidated companies remained on their respective wings even though all combined companies were still under manned after the consolidation.

At Chickamauga both Captains on the left wing were killed and Companies B and G sufferred greatly.
By the Atlanta Campaign the consolidated companies, including as many as three original designations, had only 15 men in average in July 1864.
By then it didn't much matter about seniority or placement in line of battle since regiments were the size of companies.