I just found this thread for the first time, and would like to add some info. This is in regards to bar tacks in webbing. I used to do a lot of climbing, and if you look carefully to the bar tacks sewn into the webbing and harnesses, you'll notice something. A properly sewn bar tack actually uses a straight stitch run back and forth across the webbing 4 or 5 times, then a zig-zag pattern is run over this to protect it. If it is abraided by something, the zig-zag will protect the actual stitch from wear keeping the integrity of the loop. When straight stitching, always use a stitch longer than 1.5mm, 2 or 3mm is good. When doing the zig-zag, use a stitch width that surrounds the straight stiches, with a stitch length of 0.5mm or so.
Either that, or a boxed "X" sewn twice (second one right on top of the first so it looks like one boxed "X") with a stitch length of at least 1.5mm. Both are more than strong enough. And always use polyester or nylon thread. Never cotton.