"Ich bin eher offen."
I don't think so. 'fairly' or 'pretty (much)' would match 'ziemlich' here. 'eher' is low-key, contained in this sort of context, when someone isn't very sure of or commited to something.
How are you acting towards unknown persons? - I tend to be rather (eher) vs. pretty (ziemlich) open-minded towards them. I don't think they're interchangable, since they depict different shades of the corresponding word.