My set piece selection approach is very similar to niko's.
Kicker roles will only be fixed after players are proven through testing. SA does not guarantee anything except high priority to be selected to test.
During a match, if a fixed free kicker fails and receives bad comments, I will usually replace him in that match because he likely will fail again. But I will not change his fixed role unless he fails in several matches.
Some attacking attributes also affect my priority to select players to test. Defenders' attacking attributes are often low and will not develop quickly. So I do not prefer to use them as any kicker even if they have SA.
I also rarely notice excellent corner kickers. Some of my players may provide many assists through corners. But it is not prominent that I am not aware of this until reviewing stats.
I can only remember 2 excellent corner kickers in history of my teams. Both of them were in the era that the 2nd corner always gives a goal and were corner specialists. One of them suddenly could not continue to assist because of the system change that reduce the goal rate of 2nd corner.