There's no reason to treat this any different than the imac. If you required calibration on the imac, then do the Dell too. They all drift over time. The Apple would look nothing like it did years ago whether you notice it or not. Their profiles simply aren't static, so for some measure of stability, you re-measure for an updated profile every month or every couple weeks once they're older after allowing for warm up time (at least half an hour) and keeping the colorimeter plugged in at least 10 minutes. Colorimeters drift over time just like the display. Usually they're good for 3 years or so. Sondrix is somewhat incorrect. They don't necessarily make your colors "accurate". You could profile two of the same model and see that they still don't match due to differences in aging, so which one is right? The important thing is that it's close enough. If you're going for as close to perfection as possible, you're using a known reference, and the display is part of a bigger chain. There aren't any reliable ways to just guarantee yourself perfect color, especially not with a $130 device.