I heard that Spanish puts much less value on "por favor" than English does on "please". If this is true, could someone elaborate? Are there any situations where in English 'please' is expected, but in Spanish 'por favor' is not?
This probably varies from country to country but as a Spanish (from Spain) speaker living in London I know I sometimes sound rude because I don't use please/thanks as much I should in England but also, because of that I started using more polite words while speaking Spanish and sometimes I sound too formal.
Classic example if asking for a beer:
In English "Can I have a Beer please?" doesn't sound over-the-top polite at all but in Spanish "Ponme una cerveza" (Give me a beer) can be use without being impolite.
That being said I reckon using please as much as in English can't hurt and being too polite will be better than the risk of being rude.
Your question is hard to answer because there are many spanish-speaking countries, and each has its own culture with many differences on what is considered rude, and what is socially acceptable regarding the usage of words such as please. However, as a general rule, you should consider always being polite to the people you talk to to avoid sounding rude or improper.