By implication, if you're being asked to do something that's not strictly 'your job' you've either not got much of your own work to do, or your normal job will get behind. As long as the latter recognised and I'm suitably trained and it's safe, I just tend to crack on with what I've been asked to do.
I tend to try and make my bosses job as easy as possible, and fully expect them to reciprocate, or they'll hear directly from me why they should. I don't tend to get into long debates with them, if they don't respect my help, it doesn't happen again, and they are firmly and calmly told why not. I'm quite happy to deal with any consequences that may come as a consequence, be they good or bad.
I've watched too many people spend hours stood telling others how 'busy' they are.