I will guesstimate extremely low cal things like lettuce and mushrooms but sometimes I measure them to make sure my eyes are correct in their eye-balling.

You never know when your eyed portions are creeping up to be larger and larger. I don't let things that are more than approx. 25 cals a serving slide b/c that's a slippery slope.
People are really bad about portions and most would do well to pull out a scale and see that they are overestimating what they think they are eating. You have to do what's right for you and what helps you stay on track. People often tend to call someone obsessive if they don't get the effort they put into something but we all have things we focus on and particularly if we're over/under weight it's ok and only makes sense to weigh portions for accuracy. An average weight person might not feel the need to weigh anything for portion's sake but I can tell you I have to keep track of my food for my health and if that bothers someone or looks obsessive, than it's better than diabetes.