Fit is always a personal thing. Every pair of off-the-shelf boots I've ever owned I wear a 9.5D. I measure 9.5D any time I get measured. Here's the problem though - when you get into companies that have different lasts, those standards go out the window. My foot feels most comfortable with a little room in the toe box. I don't like the sides pinching at all. I tried Redwing GTs on in a Nordstrom and 9.5D my feet were swimming. I tried 9s and they were better but still didn't feel right. I ended up getting White's SDs in 9.5D and they're exactly the way I like them - a bit of room in the toe box with no pinch, but no heel slip. Very much like I get my sneakers (9.5 or 10E).
But so much depends on how your specific foot shape fits in a given shoe - since everyone's feet are slightly different. And, everyone's preference for fit is slightly different.
Similarly, when I buy AE shoes - generic advice tends to go out the window. I buy a 5-last shoe and I get 9.5E and that's the best fit. Some people say you have to drop down a size or up a size - not me. All in all, if you never wear a 10 in any other type of shoe, I can't see it for a SD. If you wear 9.5D for everything else then 9 for snug or 9.5 for a bit loose (since these boots will be a little looser than off-the-rack timberlands) is the way to go.