I proposed to my girlfriend toward the end of the Wishes fireworks show with no special arrangements. She seemed to love it and was VERY surprised. I had been planning the trip for months, but my plans were messed up. I planned on going to MK and proposing to her on the first night during the fireworks, staying at the Caribbean Beach resort, and then going to Epcot to celebrate the next day. I was very surprised when the fireworks seemed to have been switched at the two parks for the two dates. We ended up going to Epcot the day before (and even getting in a small spat that day, which she felt very bad about, even to this day) then going to MK the next day. So, my advice to you is to make sure you have your dates and arrangements straight, just don't over plan and give things away. I actually carried a backpack with me for both days at both parks with my ring box in one of the pockets. Every time she would try to get out her lip chap or water I had to remind her not to go digging around without being obvious. It was kind of funny actually.
The only problem with proposing during the fireworks on a busy night is that you get really cramped and it might be kind of uncomfortable getting down on one knee and proposing while there are a bunch of people around. Which I did, by the way. The other people around us really got a kick out of it which made it kind of fun too. Unfortunately we had to drive home to Venice that night, a 2 1/2 hour drive. It was magical though, we listened to our new Disney CD all the way home.
GOOD LUCK!