I used to keep everything, but that faded real fast after the first 30 DVDs. Now I am more selective and delete the ones that are lower than a certain quality level. No use in keeping the ones that are blurred, or way under or over exposed, or have no special meaning.
After a quick look, I am back up to 25GB of pics to back up again (pics taken from Christmas to last night... excluding the 30 or so that I took last night playing), and thats being somewhat selective.
Some people take more pics than others (I know someone that takes a MINIMUM of 200/day, 7 days a week, and their flickr photostream shows it... they are up to 10,000+ "keepers", but then again, they are near pro and run around with a Nikon D3).
It all depends. Do whatever floats your boat, there are no set rules in photography... except the ones you want to make and break... lol.