I like to use the straight of grain for borders. It does not stretch nearly as much as the cross cut. It helps to keep the entire quilt square. But there have been times when I just can not justify the cost of the lenght for just a border.. too much left over , or I just do not have enough . When this happens I always kick myself for not having the forethought to cut the fabric for the borders first then use the remaining in the quilt.
FYI its tough to mix , using lenght wise and then another border in the same quilt on cross grain. The cross grain will not lay as flat when mixed with the lenght wise.