There's been some useful info recently added to the Server: Publish Web Maps topic in the user manual that covered something I didn't know that I think may be of interest to others.

You know how sometimes you can create a data source from a WMS server and pop it open in a window, and then depending on how you zoom in or out the display can be slightly fuzzier or sharper? Well, that can all be a result of whether you've checked or unchecked the Cache data option when creating the data source.

There's a Cache in WMS Clients section in that topic that explains why, with useful illustrations.