Sitecore recommends caching the HTML of renderings wherever it's practical to do so. Here's a quick SQL script to find View Renderings and Controller Renderings that are not marked as cacheable.
Here's a quick one that might help if you have a Sitecore instance that is not clearing its HTML cache after you publish. I encountered this problem recently. The most confusing part was that it was only occurring on one of the two content delivery servers, despite them having identical configurations.