Is the mounting screw on tripods always directly on the optical axis? I have a D5000 here and in this particular case it is. But can I trust that it's always there?
I'm assuming you mean the tripod mounting screw thread that's on the camera, not on the tripod. The answer that that is no, not always, although in most cases it will be. I've seen cameras (mainly point and shoot devices) that have the tripod mounting screw located off-center and/or away from the center of the lens.
You can count on it on all currently produced DSLRs.
You cannot count on it in general though.
There three possible positions for the tripod mount:
- Center of the optical axis: Ideal for panoramas. All DSLRs.
- Center of gravity: Most stable. Problematic for panoramas. Most ultra-zooms.
- Neither: Not ideal for anything, often better than nothing. Generally ultra-compacts.
Last year I also encountered the first modern digital camera not to have a tripod socket.
I think you'd be hard-pressed to find a DSLR that doesn't have the tripod mount very close to the optical axis, but when you start getting into smaller-format cameras, you'll occasionally see it moved to one side (ex: Olympus E-P1, Panasonic LX3, LX5). In a DSLR, this would be considered a serious design flaw, but it seems to be generally forgiven in those smaller form factors.
DPReview's detailed review of the E-P1, for instance, notes the location of the tripod mount as being off-center, but doesn't include this as one of their "con" factors for this camera.