The way my Sensei conducted gradings was in a very formal manner where he would select the Tori and Uke and instruct what parts of the grading syllabus each one should execute. In the event that the Uke was of same rank and going for the same grade, both would take turns being Tori and Uke. However it was very possible to end up with a rank beginner who may not have the sensitivity and knowledge of how to move with the techniques and make them "look good." In this case if your technique was not sound it woul appear glaringly obvious as Uke did something out of the norm.
I had an interesting time this year when the class for my Nidan grading actually fell on a local holiday, which is a time that we do not normally train. The only person I was able to contact in time for the surprise test was a white belt who had some good ukemi skills from doing Jujutsu prior to starting Aikido. Things went off pretty well, but also had some interesting parts as my Uke had to react to certain techniques that he had only seen a few minutes before for the first time ever. Of course as a result of this I had to repeat the entire grading one more time with Shihan as my Uke so he could feel my technique. Scary I tell you.