I'd like to have this option also.
My Sims dog has never slept in his bed and it's not because he won't be able to get there. Big dog beds are difficult to place sometimes because they're so big they require a lot of space around them for pets to be able to use them. I play test them every time when I place one of them (or download a house with a big dog bed), but the dogs often still refuse to use them.
It's not the same with the small beds and cats or dogs, they do use them more than big beds.
Pet beds assignment would be useful because if I will put a huge dog bed in a small Sims home I'd like the dog to actually use it. I don't want the bed to be there to use all the space for nothing.