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I've always stayed on the side of tradition and glued my bridge plates cross-grained to the soundboard, figuring it helped reduce bellying behind the bridge. But recently I started to think about the weakening effect the pin holes introduce. The bridge plate wood between the pin holes is insignificant. Essentially what the pin holes are doing is cutting a slot 3/16" x 2-5/16" out of the center of the bridge plate. The material that's left behind the bridge is a pretty puny width of relatively thin hardwood. A parallel grained bridge plate would have even less cross grain stiffness, but I am wondering if orienting the grain in the direction of the stress might actually be stiffer across the grain since the stress is spread out over a larger area.
Right about now is when the material and structural engineers should chime in. Or, those with empirical results that support or deny either method of grain orientation in bridge plates.

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I try and orient the grain of the bridge plate so that it is on a bit of an angle. That way the pins are not running along grain lines which may cause a crack (trying to avoiding the problem you mentioned). I'm not the engineering type so I'll let other comment on the strength of one method verses another.

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Josh House

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The Lowden bridge plate is positioned with the grain at about 45 degree angle. I suspect the assertion that pin holes are structurally the same as a slot is incorrect. Now it someone is going to bother with math I'd suggest that the plate, sound board and the bridge are all part of a brace assembly.

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Ken Cierp

http://www.kennethmichaelguitars.com/

Must say I agree and don't think it matters much even if there was a groove large enough to cover all the holes. I slant the grain to help do away with cracking and drill holes with a caul to prevent tear out at break through. Also slot the holes. Think those are the important points so that the string balls have solid wood to bear on and is what one should worry about.
Tom

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If I had my way, bridge plates would have grain parallel to the top grain and the pull of the strings, maximizing the stiffening of the top in that direction. Increasing stiffness in that direction reduces the "peel" action of the top that can allow bridges to come loose, helps bridges avoid cracking down the middle, and eliminates the possibility that the bridge plate could crack or chip out between the pin holes. Having the grain at an angle is helpful, but not as much. As to the stiffness across the top grain, well, the big ol' thick bridge handles that pretty well, I think.

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Frank Ford

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If you think of it as a 3 part lamination, well glued, it probably doesn't matter how it is aligned.
Pick a piece of wood that seems reluctant to split.
I like to think the main function of the plate is only to seat the string balls, not to reduce bellying, and brace the top accordingly.

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IMO, there are 3 main jobs of the bridge plate.
1. Prevent the string ball ends from eating their way through the soundboard.
2. Equalize humidity expansion with the bridge on the other side of the soundboard, to minimize stress on the bridge glue joint in high/low humidity.
3. Feather out the stiffness in front of and behind the bridge, particularly when using a thin soundboard.

The stiffness contribution across the grain of the soundboard is part of #2, but fairly unimportant. Even a really thin bridge plate helps counteract the curving tendency when humidity changes, and you can get as much cross grain stiffness as you want by making the bridge wings thicker.

3 is actually using the cross grain stiffness of the bridge plate (which is in line with the long grain of the soundboard) for useful purpose, so the thickness of the bridge plate is important there. But anything over .080" or so seems like enough to me, so there's no need to orient the bridge plate grain in line with the soundboard grain.

As for the weakening effect of the bridge pin holes, those are underneath the thick central portion of the bridge, which is plenty stiff even with holes in it, so again a non-issue.

I like the traditional orientation for humidity stability.