I would plant rye grass unless you are mulching with something that will add to the health of the soil. You can turn the rye under and it will add nitrogen to your soil in the spring.
I assume you are using the soil in your yard to fill these beds? Since you mentioned the PH.
Why not mix in some compost, cow manure, etc. and cover it with grass clippings and let it cook?

I think that is exactly what I will do. Got a neighbor with a cow barn next door. Was not sure which was best but after reading your post it makes sense to do it that way. Thanks Alan8.