Hi John,
My personal preference is to save all of my RAW data - and, once I have created suitable 'masters' of each frame-type, I will ZIP the each of the folders containing the corresponding file groups (Biases, Darks, Flats, Lights, etc.).
Then, as time moves forward, and it becomes more and more obvious that I will never be likely to require those compressed folders again, I will usually dump these off my working drives, onto DVDs (although they are so unreliable, this action has to be questionable - it may even be more effective to rely on USB thumb-drives).
Basically, hard-drive storage is now 'cheap enough' that I don't overly concern myself. I keep an eye for offers of new, high-capacity, known-brand, hard-drives as retailers move their stock on towards SSD-type drives. I (mostly) buy in 500Gb drives, and have an external USB-connected drive caddy (capable of taking either 2.5" or 3.5" drives) into which I simply drop an HDD for long-term backup.
Both my new desktop and my old desktop PCs have 4-bay quick-release caddies that carry 2.5" drives, and I managed to get my hands on some 40-odd 240Gb 'Crucial' SSDs from a local IT company that had been tasked with upgrading the entire PC fleet for a major oil company. Of the many hundreds of drive images that they cloned, these units failed whatever tests were asked of them, and were replaced under warranty - the IT company were literally going to throw the faulty units in the skip, but had to erase these first. They came to me asking if I had a simple method of bulk-erasing the data. I didn't, but offered to do the job for them in exchange for the blank drives themselves (once audited as having been correctly erased). So, I have more than enough storage space to keep me happy for years to come!