I'm creating rules for posting to twitter. However I need these rules to run every time cron runs. What is the best way to structure these rules so they send the appropriate post at the appropriate time?
Rules has a specific "React on event" under system -> "cron maintenance tasks performed". If your using the rules modules, that will trigger your rule on every cron run.
Also, if your building a custom module, and you need to call a function on every cron run, you can use hook_cron to add a function to the cron tasks as well.