If he is singing bass perhaps not, if she is singing soprano sure why not.
Really if the singing isn't low enough the roll off shouldn't touch anything that you are actually wanting to be picked up by the mic. So with the roll off engaged it will help cut down on air con noises, cars going past your house, etc.
Voxengo has a plugin called VoxengoSPAN, its a frequency analysis plugin, it can show you what range you are singing in frequncy wise. If your singing is an octave or so above the rolloff frequency there would be no reason not to use it.