It all depends on a couple of things (1) Are you staying on Property? If you are staying off property and are at the 30 day window I would most definitely go on at midnight to reserve because even then reserving the spot might be difficult. (2) Are you going during a peak time? If you are going toward a peak time at Disney then again I would definitely go on at midnight to reserve my pass. And finally (3) are you looking for any specific timing for the fast pass? If you want a certain time of the day then definitely go on a midnight so you have better chances of getting the time you want. When we went to Disney last year we were going at a moderate time, so not peak and not slow. I went on at Midnight because I wanted Anna & Elsa and then 7DMT, We had a FP party at my house, lol. We made a Starbucks run at like 9 and stayed up watching Disney movies and eating junk food like it was new years eve lol. Anyways A&E I had to be flexible with because there wasn't a lot of Flexibility even at midnight, but 7DMT still had a lot of time slots available at that point. Everything else I was able to tweak. Also just to let you know if you plan to stay up at Midnight for 7DMT don't plan on just making that FP and then going to bed bc the way the Disney system works is that you have to make 3 FPs at 1 time on a certain day. They won't allow you to just make 7DMT by itself...They will require you to make 2 other FPs that day. Even if you don't want to. And if you try to just make 7DMT and nothing else you will not be able to go in there and add the other FPs at a later date which I think is totally stupid. At least be ready to make you FPs for just the day you are planning on 7DMT, then in the morning you can make the rest of your FPs.
Anyways back to my original point of the post...if you are staying on property, are not going during a peak time, and don't care about the time slot you get then I don't think it really matters if you go on at midnight or not. But IMHO I would go on at midnight just to make sure I got a spot because I am OCD like that, lol