I learned to make endless loop strings from an article by C.R. Learn in the old Bow&Arrow magazine about 1970. His instructions were to tie the beginning and last strand ends together in a knot and then trim. Then he advised the string maker to work the string around the posts on the string jig a number of times in order to even the tension on all the strands. The only problem I've found doing it that way is that there is a knot that has to be served over, which sometimes isn't as easy as it might seem. However, you definitely do get even tension on all strands and the string is full strength and lasts and lasts.
Most endless loop instructions have you tie off the beginning and ending strands and then serving the loops to prevent the strands from slipping. That works too, but the tension may not be even among all the strands.
In my experience endless loop strings done the second way and flemish twist strings frequently don't have equal tension on all strands. Some more so than others.