I have found that although most karaoke tracks are technically stereo tracks, they very rarely utilize the stereo effects, i.e. putting different instruments/sounds into different channels, so in this regard there is little difference between stereo and mono for karaoke. The biggest exception to this rule is mutiplex, which is really just a stereo effect with the guide vocals on one channel.
Where stereo makes a HUGE difference is on your filler music, since music is so uber-produced these days and has TONS of stereo effects that you would miss in mono.
My favorite is Gwen Stephani panting from speaker to speaker throughout the No Doubt tune HELLAGOOD. Turns me on in stereo, not so much without the effect