I base my decision to speak up on the answers to the following: Will this hurt or help my child?
Will someone be harmed/waste money if I don't speak up? (For example, I was very critical of our elementary school's afterschool foreign language program, which was a joke.)
Will this jeopardize other relationships? It may be right to speak up, but the consequences may not be worth it.
Are my motives pure? Sometimes I see wrongdoing or a bad policy, but feel conflicted about speaking up because I know in my heart that I just don't like the person associated with the policy. For example, there's a selectman in our town who sent a bunch of rude, inappropriate e-mails to people he disagreed with. (He told another selectman that he was "low on the food chain" and called a town committee member "scum.") These e-mails were published in the paper, so they are no longer secret. I believe a person with those morals should not be running our town, and considered writing a letter to the paper saying so. However, his children go to school with mine, and he has a lot of friends in our small community. I chose not to write a letter and let his bad behavior speak for itself.
In the case of my kids, I have sometimes let things slide because I felt that speaking up would do more harm than good. For example, DD had a 2nd grade teacher who really didn't understand her. I was angry enough to do battle, but stopped when I realized I needed to work with the teacher or cause my child more suffering. I'm very glad I made the choice I did because my son got the same teacher two years later!
On other occasions, I've had to speak up because it seemed to serve a greater good. On one occasion, I had to tell a very good friend that her daughter would not be invited to a birthday party at our house because she had said some very hateful things and my daughter no longer trusted or felt comfortable with her. My friend didn't love hearing this (who would?) but she appreciated the chance to help her daughter's behavior, and we're still friends 4 years later.
On these boards, I speak up a little more than I would IRL not because I'm a wimp (although that can be true!) but because I am far better at writing than speaking. I imagine this is true for many people.