If you've already got them, use them. If you don't like the way they perform you can always replace them. I'd probably try to mount them horizontally, as that's most likely how they were designed to be used. But if that is not an option, use them vertically, at worst, the dispersion will be less than ideal. Surrounds are more forgiving than fronts, in that they generally get a lot less sound sent to them than the fronts do.
Jack