4x6 prints are a whole different thing. I agree you won't find noise in them, even at high ISO, but a page-size or larger print is a much tougher test, not to mention projecting the image on a screen. If all you're doing is 4x6 and/or displaying on a small screen, then by all means use the highest ISO setting and don't worry, but otherwise be careful.
What you want the photo for, and your expectations make a huge difference. After I got my 5D Mk III, I saw some forum that said ISO 160 was the native ISO of Canon cameras and ISO 100 would have more noise. Well, it's a fairly easy test. A dim light, a tripod, a target, ... So, I did a test set with the 5D and also with a Rebel T2i (550D) which is my favourite due to it's lighter weight. Results are here:
CameraClicker: Digital Noise 5D Mk III
My Rebel has been around the world. Here are a couple from the 2010 World's Fair in Shanghai, inside the Canadian Pavilion. These were at ISO 6400, wide open for that lens and minimal shutter speed. ACR was used to convert from raw files. Noise reduction was left at the default 0 for luminance and 25 for colour.

and
Sometimes you need a really fast lens, big aperture and high ISO just to get enough shutter speed. There are times when a flash won't work, either due to distances involved, lack of permission, or like here, inside a bicycle powered movie theatre. So, get the shot and don't worry so much about noise. I took a 50 mm f/1.8 with me on the trip but most of the time I relied on my Sigma zoom lenses and the stabilizer in the 18-250.
If you really care about the noise, use DxO to process the raw files. It removes amazing amounts of noise. Adobe Camera Raw's noise reduction is pretty good too.