Our psychologist at the time (DS was 7 I believe) told us to bring the kid so we did. I did not know she was going to share the number with DS at the time, and would have asked her not to. However, she insisted that kids who are this much of an outlier need this information about themselves to develop a healthy self concept (though she admitted she'd never met a kid with abilities this high who was so comfortable with them, and I was so glad we'd done something right, both in our parenting and on our choice of school - DS10 has loads of other issues and hangups, but he is happy and confident in his intellectual abilities).
I still believe that a ballpark figure or range would do. At least for a kid under 14. I'd hate for numbers to come up in a conflict in his gifted program at school, or with siblings. Luckily, he has forgotten the exact number and when he asked, I gave him a general idea of his LOG without mentioning a number. If he asks at anytime after turning fourteen, I will turn it all over to him, because I agree by that point the information belongs to him.
With an 11 yo I'd be on the fence. Agree I'd ask both the tester and her, but still think would prefer to give (or have the tester give) a general idea of LOG, with a promise that she can see all info later when she is older.
Agree that understanding about cognitive abilities being a tool is so much more important.