In Catalog settings there is a tab with settings to control LR automatically discarding previews.
If your files have been copied from your laptop to your master working drives and you want to clean those files off the laptop you don't need to keep the preview files.
When shooting in the field, here's what I do:
1. On my laptop create a new Lightroom catalog, in a new folder on the desktop. The folder is named for the trip and will contain all my image files, a Lightroom catalog and the Lightroom previews package.
2. I then do all my downloading, DNG conversion, keywording, metadata and field edits working in the temporary field catalog.
3. When I return to my studio, I connect the laptop to my studio desktop machine via Ethernet. In my master working library, I use the Import from Catalog command, which simultaneously copies the image files to my master drives and imports them into my master LR catalog.
4. When everything has been copied and backed up, I delete the trip folder and all its contents from the laptop. The temporary LR catalog, all the preview files, and the image files themselves are deleted, freeing up my laptop for the next trip and a new set of files.
I've found it's very useful (and easy) to work with multiple catalogs, as needed, while also having a master catalog that contains my entire image library. This allows me to do lots of work on the road and easily integrate that work into my master library.
This "packaged" approach makes it much easier to keep track of files and make sure they are handled properly and backed up frequently. Sometimes I burn DVDs in the field, and I can usually get a day or two's worth of images onto a single layer DVD.
