When a doctor asks me how I'm feeling could I answer with:

I'm not sure.- I know it's vague but let's say I really don't know or in too much pain to give him a straight answer.
A useless response.
Students: We have free audio pronunciation exercises.
I guess that means it's okay to say it but doesn't really help the situation?
When the taxi driver asks your destination, do you say 'I don't know'?