Should I agree to teach a class for my supervisor even though I don't get any acknowledgement for it? I'm not contractually obligated to teach in my postdoc position, but she just asked me as a favor... I don't mind of course, but I'd rather be formally acknowledged as co-teaching on the syllabus, etc.  It's good for my CV, etc.

Is it expected that I comply with my supervisor or am I within my rights to expect formal acknowledgement?
This already has too many answers, but I can't help myself: academia entirely runs on people doing stuff they are not contractually obligated to do.  This generates a lot of problems along the way, but that is how things stand.  "It's not a formal requirement that I do this" is not likely to be a satisfying answer for your supervisor.  If the amount of lecturing is not a huge burden, and you feel like it will not make a huge dent in your productivity, then it is probably a worthwhile thing to do (if only as a favor to your supervisor); if you feel like the amount asked is a large burden, see if you can negotiate to reduce it.  I also agree that you want to get something in writing.  I would phrase it as "I want to be sure I understand what needs to be done for the class.  If it's writing, it's less likely I'll forget or misunderstand," rather than making it seem like you are suspicious.