What cache are you talking about? Browser cache can be deleted at any time (I do mine once a day when I quit the browser) - it used to be that caching helped load websites faster, but with today's faster internet connections as well as faster processors, etc, that really isn't relevant any longer.
If it's not the browser, what are you talking about (screenshots would help).
On the whole though, videos, music, etc take up more space on your hard drive than the cache. Please remember: you need an absolute minimum of 10 - 15 GB of space empty and available at all times for the OS to function properly.
In backing up my iPhone I'm running out of space on the HD so I'm looking for any thing I can get rid of. Any tips on that?
Yes! Put your space hogging files (videos, music, graphics, pics) on an external hard drive permanently. After you've done that and made sure they are working, delete them on your internal hard drive. As an example, how to move your iTunes media folder:
Do not add anything to your hard drive until and after you have gained a minimum of 10 - 15 GB of free space (add to that whatever else you are wanting to download or back up). If you continue and run out of space, your Mac will not run.
you are going to have to delete, move or archive your data to regain at least 15-20 GBs of empty hard drive space.
Have you emptied your iMac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Other things you can do to gain space.
Delete any old or no longer needed emails and/or archive older emails you want to save.
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive or delete any old documents you no longer use or immediately need.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
You may have to Purchase an external FireWire or Thunderbolt hard drive to move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.
If you have any other large folders of personal data or projects, these should be moved, also, to the external hard drive and then either archived to disc and/or deleted off your internal hard drive.