Media Cache files are transient so you do not need to keep them. I have a folder called 'Hold4Delete' that mirrors my disk structures. Then, when I'm doing any significant tidying up, I move items for deletion into that folder structure. If after a month nothing has gone wrong I delete them properly.
Insanity is hereditary, you get it from your children
As I understand it Media cache files are only used when you render any part of the timeline. If you deleted them then only this rendering will disappear and the red lines will reappear.
It won't affect your project that can always be rendered again if you really want to preview it again.
Rendering is done anyway when you eventually share it to anything. It might just take a little longer.
Someone correct me if I'm wrong!
There are several PrE caches. All of the caches can be deleted without issue -- but don't delete the capture video and audio directories without first verifying they can be deleted. I would recommend deleting the cache directories with a new O/S install because some of the cache entries may just be eating up space or may be even duplicated with the new install.
The side effect of deleting the cache will be additional time in opening up the project because the cache will need to be rebuilt or poor performance because the preview caches are not available.
For future reference, there seems to be two types of cache and a holding directory...
a) Media cache
b) Project cache
c) Capture holding directories.
The media cache is not project specific and will be shared between projects. This consists of the media cache database (under 'Media' in Preferences) and the actual media cache (under 'ScratchDisks' and 'Media Cache' in Preferences). Files are created here when PrE index and conforms media items -- usually this is when a new media item is addded to a project.
The project cache (under 'ScratchDisks': 'Video Previews', 'Audio Previews', and 'Disc Encoding' are specific caches for a project.
The capture holding directories (under 'ScratchDisks': 'Captured Video' and 'Captured Audio') are holding directories for captured video and audio.
NEVER delete anything under the catalog directory. Many items cannot be rebuilt.