When is really necessary to visit field to observe sample collection process?
After being a lab tech, just because they should report everything, doesn't mean that they are doing exactly what you think/wish. I would say it is important so you can see if the process of sample collection and processing is exactly what you need for your thesis. I had to follow a written report on how to feed a certain insect, after a few nights the post doc came in and realized there was something missing from the report and we were feeding the insects one the wrong side of the leaf. May not happen to you, but I think its always good to verify.