Excellent advice! I have seen people grade too fast, but also too slowly - this creates anomalies which cause problems as well.
Minor quibble - Aikikai grading criteria are minimum 1 year to nidan, 2 to sandan and 3 to yondan, but with practice days requirements as well:
http://www.aikikai.or.jp/eng/gradingsystem.htm
Your stated minimum are the more practical levels.