Retina screens use four (2x2) more times pixels to render an image and thus, you must provide them with higher resolution images to actually display best. To render correctly on Retina screens, you should use a "double-sized" image. For example, if you want to display a 320x240 image, you should ask for a 640x480 image and force its size down to 320x240 using HTML/CSS. Although it will require a bit more bandwidth and thus will be slightly longer to download, this will make sure all screens will display images at the best quality.

Should I do it?

That's up to you to decide. The trade-off is speed or quality. Decide which one is more important to you and act accordingly.

You can use your Web Analytics software to help you decide. Do you have a lot of Retina users? If yes, you should consider it. Do you have a lot of mobile/3g users? Maybe it's not such a good idea then.

Every situation is unique, so you're in the best position to chose what best suits your users.