One of the best spots in MK is the Plaza Rose Garden - perfect view of the castle behind you, but the crowds are way less obvious (bushes and trees kind of obscure them).
It's such a great photo spot that Disney almost ALWAYS has a photopass photographer at the entrance (as quoted by Disney Guest Services). If you're able to tip off the photographer beforehand he or she can let you inside the gate and onto the grass!
My brother just proposed last week this way. We said we wanted a big group photo and the photographer played into it SO well, saying she couldn't fit us all into the frame since there were 9 of us, so she was going to let us onto the grass. So we all posed, got some shots, then as we started to break up and walk away, he grabbed her hand and got down on one knee.
Then we'd gotten some pixie dust from Bibbidi Boppity Boutique for the kids to throw on them (it was a mess, but great!). You can also get some at the Barber Shop on Main St. (we just took our own baggie and asked if we could have a little for the surprise coming up.)
The photog said she'd been hoping to get to do a proposal that day and she was glad we'd approached her to give her some warning. She said lots of people kneel down on the path, but if she knows ahead of time she can let them on the grass.
Oh, and THEN we had the wedding Mickey and Minnie hats that we'd ordered earlier with Mr. and Mrs.---(last name)--- embroidered on them, so they were waiting afterwards. It provided a nice option to go and pick them up, instead of milling around like, 'uhh, now what'.
I can post pics if you're interested.