Read the replies with interest - Lianne, I still say go with how you feel about the grading yourself. Yes, you may have to compromise vis a vis your sensei, but you obviously have ceratin issues with your aikido you want to address prior to grading, so go for it, I'd be very interested in what happens.
As for some of the other "advice", which basically boils down to "do what sensei says you 'orrible little oik" - nah, never been a fan of this one. Firstly, it implies that there is going to be some sort of struggle between you and your sensei. Secondly, it also implies your own opinions about yourself and your training should be subsumed into those of the dojo (and it's prophet on earth, the sensei).
Well, firstly, I have a person who teaches me aikido, I do not have a acolyte relationship with this person. While I am more than happy to accept their authority regarding teaching aikido , their dojo etiquette etc., a grading is different as it can be an intensely personal experience. I have seen many aikidokas driven out of a dojo because of grading mania and find the more successful teachers are those who lead by example rather than fiat.
The second main argument (which was disturbingly present) is false. A person is ultimately training for themselves, not a group and hopefully not for their sensei. To (badly) paraphrase the Prisoner - "I am not a coloured belt, I am a free man"