This isn't a mantra but it is a starting point.  360 degree feedback works best when it is development focused.  When it is assessment based and part of appraisal then you have to be careful that you can get the results you require and that the recipient gets something useful for them.

If your 360 feedback process is development focused then all participants can give feedback openly and straightforwardly safe in the knowledge that the intent is to give the best feedback that will give the recipient most chance to improve.

If the 360 feedback process is appraisal based then all respondents have a filter operating as they complete.  They will generally continue to seek to be fair but the manager and the self responses in particular will be influenced by the appraisal.  Direct reports will be aware of the process and considerate of the impact of positive and negative responses.  None of this precludes using 360 feedback as part of appraisal - many of our clients use it very successfully.  But it does remind you to think it through carefully, get your communication and training right, and build in some failsafes.

Brendan