I have a lot of trouble figuring out how to handle certain situations at work too. I'm never sure when I'm overreacting, and when I'm standing up for myself. My best advice would be to really know the guidelines for your job well. If it's in your job description, do your best to complete the task, but if it's not, you should be able to say that you're sorry, but you're unable to help them with that. And if it makes you feel unsafe, just don't do it.
It's really hard for me to make those judgement calls though.