A little background on my dog... He's Stafordshire Terrier and German Shepard mix. Insanely strong, and loves to pull to go meet a new dog or say hi to someone. His best friend is a 200lb Great Dane, who gets pulled around room while playing tug-o-war. If there is water anywhere around, he is diving into it. Since I'm in So Cal, he gets to go play in the ocean during the summer. Every nightmare situation you can put a collar through, is his normal play time.
I typically make martingale collars as they help with controlling larger dogs that like to pull, and also keeps them from slipping out of the collar. I use 7 oz veg tanned leather, glue, and hand sew the collars. Typically I buy double shoulders, as it is more cost effective. I do sew the across the D and buckle, and have yet to have a problem. I know it could be an issue, but if my dog hasn't popped the D off it must be something to be aware of but not worry about. The biggest issue I've had is the dye starts wearing around the edges, but then again this is after the collar has gone through 6 months of hell. I think I've possibly fixed the issue by following the the finishing edges thread...