It's a grey area at best. Apple never intended for anyone to keep the DVD afterward, by downloading the CD images you're essentially getting 2 copies of the software. I myself don't worry about it too much, on the other hand I have a very light handed approach (specifically, I don't care) but it's not me that decides what happens here.
I only keep the CD copies these days, same software but a broader spectrum of hardware it'll install on.