Asymmetric DRAM on Ivy Bridge and later?

I have seen a number of current generation (Ivy Bridge and later) laptops being sold with asymmetric memory configurations.  In other words, the two memory channels are not populated identically with DRAM.  For example:

• 4GB in one channel and none in the second channel
• 4GB in one channel and 2GB in the second channel
• 4GB in one channel and 8GB in the second channel

Will such asymmetric configurations result in some loss of system performance?
What are the memory configuration 'rules' for maximum performance?

Thank you!