Re: How ready should you be for a test?
I'm not an expert with tests nor aikido but it depends on what you mean by "polished" techniques.
I just note that when you go observing tests for 6th kyus, 5th, 4th, ikkyu and so on going to higher dan grades, you observe differences in how "polished" the techniques and movement are as you go from lower level to higher grades. and its not only in the techniques, technique is just a part of a good test IMO. Other important factors are in play as well like zanshin, posture, confidence, proper kuzushi, clear/strong entry, crispiness/economy of movement, the finer details of the technique, calmness of mind, centredness, clear understanding of kihon and many others which are clearly refined and ever present in the higher grades. Having very good understanding of these factors would lead IMHO to your movement being "polished". You need to have these factors at a level which suits the grade IMHO which your teacher can be able to ascertain.
If the lower grades I know have more of the above factors than myself and know better than I do, then I think I am not ready to grade yet.