This post, To grade or Not to Grade...That is the Question, provides one of the first honest presentations of what is behind this question that I have seen.
Here are my thoughts today:
Gradings anywhere are about facing knowing what someone else thinks about you, and facing someone else's assessment of you is ultimately facing yourself.
What did the founder say was a goal of aikido? Masakatsu agatsu - True victory is victory over oneself.
It is true that your skill, talent, understanding, and so forth, are your own whether you test or not. From that perspective, rank means nothing. However, rank does not truly mean nothing until the possible pride or shame that may arise from the test and the wearing of your assigned rank means nothing as well. Note that this includes comparing yourself with others. If you do this, then rank still has meaning to you; you create rank every time such a thought arises.
If rank really means nothing to you, then you neither crave rank nor do you have an aversion to rank. In that case, you may as well test.