Paul P
Rain
I know some people think it's heresy but I also check my mixes on the Macbook's own speakers.
I cringe every time my kids and their friends play music through their laptop speakers.
It seems that as long as the music is recognizable, the brain can do the rest.
I would never listen to music like this, but people do it all the time. At times my wife will listen to music on her iPhone - through the internal speaker

. Rain also mentioned earlier about having a "mono cube" dock system in his gym that he will reference. These are all very good reasons why mixes should be...
A: Checked in mono (mono dock systems are sold all the time)
B: Checked in the worst possible situations (such as laptop speakers)
Key word in both is "checked". You don't mix to those situations, but you fix any problems that get revealed by those situations. This assures your music sounds the best it can even in the worst listening environment.