I'd agree with leon, that shoot what works for you.

However since you asked, my $0.02 would be:

If a bridge looks horizontal then you're standing in the middle of a river (or a train track). I don't like shooting stuff square on, and would move to get some perspective going. Then the lines of the bridge draw you into the photograph.

Foreground/background and subject are all abstractions. A photograph only has a subject when someone looks at it - the camera just records the scene. You can either choose to make one aspect stand out, or give the viewer more choice.

A single branch or leaf is an intrusion, but a tree or bush is simply another part of the image. Foreground objects place the photographer (and viewer) within the scene, and gives a sense of place. If used well then they're good. Just make sure it looks deliberate, rather than accidentaly allowing folliage to intrude on the edges of a scene.

Like I said at the begining, there are no rules. I'm no expert, but thats what I would do in those situations.

Ian