What is your performance rating scale? Does your organization have rating distribution guidelines?

We also have a 5pt rating scale, which we review twice per year. We assess a few different points: Communication & Feedback, Teamwork, Adaptability, Time management and Proactiveness, Problem Solving and Quality of Work, and Accountability.
We then cross this with another set that is a matrix of probability of churn (1-3) vs Impact of churn (Normal Employee, Important Domain knowledge loss, Risk of losing potential Rockstar, Important for future growing Business, Impact on team structure, OK to lose).
These 2 sets help us have a better understanding of where an employee is and how to manage them

We have a 5 pt rating scale (often misses objectives, Developing, Performs well consistently, exhbibits excellence, always exhibits excellence) with a distribution guidance of 5-5-60-20-10). We have a midyear with no ratings. Distribution is a guidance and not forced. 

5-point rating scale.  No forced distribution. 

5 rating scale (1 being the lowest, 5 the highest) . We have guided distribution but do keep an eye on the ratings per the impact on the allocated short term incentive plan where we do budget balancing (there is a budget for a reason....) so in the end managers will need to recalibrate to stay within the budget. 

We are transitioning to a 5-pt scale (from a 3 pt scale) to allow our leaders greater space to differentiate employee performance which links directly to their pay increase. We "recommend" a bell curve distribution.