For my work, a 13Mpxl raw becomes a ~300MB PSD. I can fit 6,990 of those on a 2TB drive. A 2TB drive costs $100.
If I were to think about going through that drive, once full, to delete stuff, and spent 10 seconds on each image, it would take me 69,900 seconds to go through the drive and cull them all. Buying a new drive costs, maybe, 3 hours of billable time for the average retoucher... That's 180 minutes, or 10,800 seconds.
It is literally not worth your time to delete old files (by a factor of 7 in my case). Maybe if you only made $4 an hour retouching, or if your PSDs were larger than 2GB each, you'd find some savings in using your time to sift through a hard drive but as it is, no chance.
Just be sure you organize them logically when you create them, and let them all come along for the backup. Unless you're shooting lots of video, drive capacity will outrace your ability to fill it.