If you make two copies to CD or DVD from the same batch and find later that one has gone bad there is a good chance the other will be bad also. I’ve saved things to multiple CDs and DVDs without other backup but I’m fussy when I do it. I use two different brands and run Nero CD-DVD Speed on the higher quality disc to make sure the error rate is low. There are CDs I burned eleven years ago that have been in my truck changer all that time that still play. So they can be reliable.
I’ve learned from running CD Speed over the years that a medium burning speed (maybe 24X for CD) usually has fewer errors than full speed. Most burning programs let you set the speed. Some people feel that 2X or 4X (extremely slow) is more reliable, but I haven’t found that to be the case with any of my CD burners. I’ve had only a couple DVD burners and that isn’t enough to make a good judgment, but medium speed seems to have lower error rates. The X speed equivalent is a lot higher with DVD.
It is best to use two different forms of storage if you are going to erase the source from the hard drive. I use an external hard drive and either CD or DVD. Memory sticks are reliable if you store them in a safe place, but you should also copy to CD or DVD as well.
Whatever you elect to store your photos on be sure to thoroughly check your storage copies before deleting them from the computer HD. Just because one photo looks fine doesn’t necessarily mean they all are. And don’t trust just the thumbnails – it is possible to have good thumbs and bad full sized photos.