Doesn't matter. But it's a little weird when they're shorter than me. Just saying. Other than than the weird factor I don't mind what height they are. My bf is 5'10 and i'm 5'7 so it's like perfect. I don't have to bend down and reach up on my tippy toes. It's perfect.
I like how I can hold a girl shorter than me into my chest more easily, wrap her up with my arms. That said I've never had a girlfriend the same height or taller than me, so I can't say for certain. I would feel slightly odd if I had to lean up to kiss my girlfriend though :/