No, it doesn't matter it will be just fine no matter where you plug it in. The mouse only uses 1.5MBPS, the keyboard is 12MPBS and the ports in the iMac are 480MBPS.
whoops.. I completely mis-read your question. I thought you wanted a different way to plug in your mouse so you were not waisting a port on your keyboard. My bad, I appoloize for my useless post and thank Ben, for picking up my slack on that one.. Sorry.
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