Even though storage space continues to bceome less expensive I'd still like to reduce the amount of material that gets archived for the long term. Is it prudent to dispose of Preview files? What else can I safely get rid of?
I'm sure there are varying opinions on this topic.
Yes, you will just need to recreate them by rendering your project if you need to reedit in the future.
The things you need to keep are the project files (including PP, After Effects, SpeedGrade, Audition, etc.) and media (including video, audio and stills). Everything else can be deleted.