12"x8" is 3600x2400 pixels @ 300 pixels per inch (what DSCL print at).
Let's say you wanted to print at 12"x8" and your crop was exactly 3600x2400 pixels. That would mean that every pixel from the camera represented one dot on the output image. If your crop is not exactly 3600x2400, you have two options: either leave it as 1 printed dot per pixel from the camera and it will print out smaller (if the crop is a smaller number of pixels) or you can resize it to 3600x2400 and still print the crop but at 12"x8".
The key then is to understand the pixels per inch you will be printing at. If my crop is exactly 3600x2400 and I'm printing at 12"x8" and my print lab prints at 300 dpi (dots per inch), then I am printing at 300 ppi (pixels per inch). I.e. one pixel in my image represents one dot on the print paper. If my crop is actually 1800x1200 and I print at 12"x8", then my print lab is still printing at 300 dpi (because that's how they work) but my 1800 pixels are being printed at 12" which is 150ppi - or 1 pixel becomes 2 dots on the paper. If I'm 900x600, i'm now at 75ppi (1 pixel -> 4 dots printed). Obviously, the lower the ppi, the lower the resolution of the resulting print will be as you are spreading fewer pixels over the same area.
The key here is to know how many ppi you can get away with for an acceptable print. I haven't done a lot of testing here, but I have got acceptable (i.e. for a family member who wanted a particular snapshot printed but wasn't looking critically at the quality) 10"x8" print from a heavily compressed Facebook 720pixel image. That was printing at an effective 72 ppi and the image was from a compact camera and I was very surprised at how well it came out (it was a 100kByte image I started out with!).
Personally, I try to keep my ppi as high as possible - in the 200 - 250+ ppi region. Given I like 18"x12" (or 5400x3600 pixels @ 300ppi) prints that means cropping as little as possible from a 5D2 file and getting the subject as large as possible in the original image.
So... in short: it depends! Crop in too much and print the resultant crop too large and you'll be disappointed. The problem is in knowing what your level of acceptable ppi once printed is and only you can determine that (although the higher the better clearly!).
