Possessive 's after S - Pronounciation

Okay, I will give my shot at it, but I will somewhat contradict what I said above due to having thought about it more:

Spears: sometimes takes /ɪz/
Williams: does not take /ɪz/
Jones: sometimes takes /ɪz/
Gonzales: takes /ɪz/
Roberts: does not take /ɪz/
James: sometimes takes /ɪz/
Chris: takes /ɪz/
Francis: takes /ɪz/