Is it necessary for both memory slots to be equal in size?

Hi all,

I'd like to upgrade the RAM on my mid-2009 MacBook Pro. Do both memory modules have to be the exact same size (ie: 2 2GB modules in each slot) or can they differ (ie: 1 2GB module and 1 4GB module). I understand that Apple recommends having the same size for both slots, but was wondering if it hampered performance (or something worse).

Thanks for your help!