I found this excellent article on the merits of straight grain, cross grain and bias binding. The whole article is well worth reading but the following really got my attention... it makes so much sense, yet never occurred to me.
"Bias binding will wear longer than straight binding so in a bed quilt that is getting lots of use bias might be the better choice. The straight binding will have only one or two threads actually on the edge of the quilt. These few threads in the fabric right on the edge of the quilt will wear faster. Bias binding has stretch and flow and moves at the edge and won't wear as quickly."