To pipe up or not to pipe up?

Is a direct approach the best? Or should we go along to get along? I tend to be The World's Biggest Weenie, so when someone does something that upsets me I tend to not say anything to that person, and then I go bitch about it to John. Take a recent trip to the vet, for example. I took Luke in for his semi-annual appointment recently and when the vet tech asked me his weight, I told him (97 pounds). The vet tech gasped and then proceeded to lecture me for several minutes on the importance of keeping one's dog's weight under control. First of all, Luke is a "giant breed" and he's all muscle. I defy anyone to find an ounce of fat on that dog. Second, the vet (who has known Luke since birth) has told me that as long as his weight is under 100 pounds, he's fine. Third, he had just eaten breakfast. When I weighed him later in the day, he was 95 pounds. No shit. So, no need for a lecture: the dog is fine. But instead of saying any of that, I simply nodded my head and patiently waited for the vet tech to be done. I wish I could be like my friend Dana (hi, Dana!), who has the guts to speak up when something or someone disturbs her. But I don't seem to have it in me. What about you? When a friend says something unkind, do you talk to her about it? What about if you're standing in line and someone cuts in front of you? When is it acceptable to pipe up, and when should we just let it go?