It doesn't really matter where your horse's ears are, just so you know that he is paying attention and obeying you. My horse always has her ears pricked forward, listening to other things until I cue her. Then one or both ears go a little back or sideways for a moment to tune into my voice.
But, I know a few horses that do a lot of different things with their ears, so anything is fine except for ears laid flat back on the head. Then it's not so good! They are either being aggressive, resistant, fearful, and/or defiant.
Edit: Sometimes Brandy will have her ears flat out like Dumbo, which tells me she is either really bored or overly relaxed.
**I must not forget to thank the difficult horses, who made my life miserable, but who were better teachers than the well-behaved school horses who raised no problems.**