Throughout the editing process, we create a good deal of media files by capturing off of tape, importing files, Consolidating, or Transcoding files linked by AMA, and of course by rendering effects. It is likely that you will want to determine what media is being used by your sequence(s) and what is not, so that you can delete it. This will not only free up drive space but also in some instances may even improve the performance of the system because the Media Composer software will have fewer media files to keep track of.
Deleting unused Precomputes is something you may want to do every few weeks. Deleting unused clips is generally something you'd do in the very late stages of a project in order to make room on the drives for some new footage that has arrived.
When deleting media, slow down, take your time, think things through, eliminate distractions, and don't do it when you're tired.