Do modern browsers cache ajax responses?
According to HTTP/1.1 specification browser HTTP cache key is a combination of a request HTTP method and URI. Browser doesn't take into consideration how request was made:
The primary cache key consists of the request method and targetURI. However, since HTTP caches in common use today are typicallylimited to caching responses to GET, many caches simply declineother methods and use only the URI as the primary cache key.
So if requests that you perform are cacheble you still need that header value. Check HTTP Caching MDN page and Which webbrowsers use http/1.1 by default? question for more details.
Also you may need to add another headers for IE specifically. Check Make IE to cache resources but always revalidate question for more information.