I wouldn't opt my child out of it unless my child found it extremely stressful and it had a negative effect on his day to day functioning.
For two reasons:
a) I think standardized testing can provide useful information in terms of how your child tests and what he does and doesn't know. It can be a useful diagnostic tool in terms providing information to you, your child, and your child's teacher about what he has mastered and what he needs further work in.
b) I wouldn't give my child the message that s/he does not have to do what s/he doesn't like to do just because s/he doesn't like to do it.
Personally, I wouldn't teach my child that s/he is above the rules that everyone else has to follow and abide by, unless there were extenuating/mitigating circumstances and/or a very good and valid reason.
If I were against the tests in principle for EVERY child, I'd advocate against the tests.
But if my child was in that school and every other child in that school was expected to take the test and it wasn't a hardship on my child to take the test, I'd teach my child that sometimes you gotta do what you gotta do whether you like it or not. I wouldn't teach my child that s/he was somehow 'special' and deserving of 'special' treatment.
I'd rather have my child take a test than turn out to be someone who thought they somehow didn't have to do the tasks or follow the rules that everyone else had to do or follow.
I personally feel that I got good and valid information from my childrens' standardized test results - even if it was only to learn whether or not they were good test takers.