Grading, in my opinion is a means of justifying the end. As long as you have trained hard in your day-to-day training, then a grading is simply another day in training, albeit under a closer scrutiny. Generally there are 3 ways you can go about taking a grade.
1- Your sensei calls for your grading.
In this case, as with my dojo, my sensei often points out to an individual, and tells the fella that the person is due for the next grade, and that is it. As long as the Aikidoka do not screw up too badly, chances are the person will get the grade.
In this case, your instructor has assessed that you are suitably proficient to meet the next grade and calls you to grade, as a formality.
2- Your time is up for a grading
This is more of an academic approach, like as in a university, where an exam will take place at the end of a semester, ready or not, like it or not. In this case, you have to be present to take the exam, and the certainty of passing is less of a guarantee.
3- You asks to be graded
In this case, you proactively asks to be graded, which is a rarity, but my sensei did mentioned that some of his senior belt, asked to be graded and promoted from 2nd Dan to 3rd Dan, to which of course my sensei rejected. The particular individual never turn up for class any more.
This is a rare as the oriental martial arts always practice humility, sometimes to an overbearing level. More often than not, we wait to be called for grading or we wait till out time is up for it, as asking for a grading appears to be rather cocky and self assuring.
Either of these 3 approaches, a grading is simply a measure of gap. Either your skills meet and exceed the grade, hence, the necessity presents itself for you to take on a grade of higher level, or you are asked to take the advance grade first, then you polish your skills up to meet the level of the advance grade.
The truth to the matter is, if you examine closely the grading scale, relative to your skills, there will be some discretionary gap. Rarely is such that a person’s grade is closely matched with the person’s skills, and being highly graded doesn’t make you invincible, in fact , it should make you, more human.