This is one of those “unwritten rules of kendo”.
Don’t ask your instructor for permission to grade.
There. I said it. Just don’t do it.
If you have to ask, you’re probably not ready.
If you’re ready, your instructor will let you know.
If you don’t think you’re ready and your instructor tells you to grade, don’t second guess them. Be like Nike and just do it.
That being said, there are plenty of ways to figure out whether you should grade. Just ask indirectly. Ask if your technique is improving. Ask what you need to work on specifically to attain the next rank. Ask your sempai. Watch what the people who are at the next level are doing and figure out if you’re anywhere close to doing what they are.
Most importantly, pay attention. Know what you’re doing and how you’re doing it. Know what you need to work on. Ask for feedback on your technique. Keep striving to improve.
Do all that and you’ll know whether you’re ready to grade without asking.