YES! They're so perfect for each other, aren't they? Sebastian needs someone who isn't boring(sorry, but Ciel seems to be probably the most boring person in the universe to be in a relationship with. I feel bad for Lizzie) and who shares enough traits with him while still being a very different person.
Grell needs someone to match her inner fire. William would, I think, only contain it and she would end up feeling very trapped. He is so strict and unrelenting. He never shows any emotion to her but annoyance. That would be soooo disheartening after a while! Sebastian, on the other hand, wouldn't contain her. He might steer her destructive tendencies towards something a bit less destructive, but I don't think he would make her feel trapped.
And finally, we all KNOW Grell likes the fairytale romance and needs someone who will let her be deeply passionate about him. Sebastian is the PERFECT choice.... AND THEY ARE JUST SO CUTE!