Sometimes when you use info-hooks to collect information from a arbitrary number of modules, or perform some other expensive or "unknown cost" operation, you'll start to feel the need to cache your results. If your function will be called several times during one request caching in a static variable will take off some of the load, combining this with the Drupal cache could help even more.

Caveats: you probably know this already, but as with all caching it's only effective/sensible to cache stuff if the cost to calculate the results from scratch exceeds the cost of retrieving the results. This is especially important if the database backend is used for caching. If APC or another fast, memory based caching backend is in place, the list of stuff that could benefit from caching grows a bit longer. The other consideration is hit-rate - will your function really be called often enough for it to be necessary for it to be cached? If your cache is invalidated regularly because of writes this becomes even more of an issue, if the cached never is used, or at least not used enough to justify the cost of the actual caching, you will just have written unnecessary code, or in the worst case unnecessary code that just makes your website slower.

Now, the reason for this post: sample code & potential best practice here at Good Old. The following snippet collects information through module_invoke_all() and then lets other modules alter the very same info in a subsequent drupal_alter(). The results is cached in both statically and in the Drupal cache: