Nice guide! A few recommendations about specific placement on your images. Descriptions are based on left/right/top/bottom of your pictures.
Gravel Pit A. A temporary location that will guard the health below the point as well as people running to the point is in the corner behind the shed, right above your "C Tunnel" text. A demoman can easily clear it out, but you'll keep people off the health for a little while an most people will run right by the sentry without looking at it. You can also stand on top of it and do a crouch jump while placing your teleporter exit on top of the shed, which is the most hidden location for a teleporter exit at A.
Gravel Pit B. The sentry on the big ramp coming in (to the left of the point) should be moved to the top (image-wise) of the ramp. On the bottom it's exposed to a sniper on the right exit through the window. Plus, you can heal it from a dispenser in the corner if it's towards the "top" without getting hit yourself (except by stickies or from behind).
Gravel Pit C. Blue can often get a sentry in the left tunnel leading in, under that ramp. It can hit the entire side of the tower, which makes it easier to cap the point. This sentry won't last long, but can do a lot of damage in a little amount of time. Same with sneaking a blue sentry onto the wall of the red spawn or dropping a level 1 at the foot of the tower (the first ramp up) while continuing to run up the tower. It can often build itself all the way up while you draw attention elsewhere, and will protect you if you make it to the top.
Dustbowl Stage 2. An excellent sentry and dispenser location is directly below the O in "One", pointed across at the far shed. It should be in the corner to the left of the door, but outside. It can hit people on the CP as well as people coming around the far side of the shed. You're vulnerable to stickies and uber pyros here as well, but seriously few locations aren't. It's a hard location to take if you have teammates playing near the shed, a sentry above you next to the point, and/or a heavy sitting on your dispenser keeping enemies from coming up behind you.
On the same stage, an excellent blue sentry position is anywhere in the 2nd cap point area. I'll often wait for my teammates to go in uber, which draws the attention of all of the enemies and then I'll drop a sentry pointed right at the CP in the tunnel (right below the metal + health packs). Most enemies focus on the people on the point so it's relatively easy to get a level 1 sentry up which can help clear enemies off the point while you finish capping.
Same thing on Dustbowl Stage 3. Blue can often drop a sentry on either ledge beside the final CP (the raised one to the right and the one around the water tower) while teammates are trying to cap. If you can drop one, even if your team doesn't cap right away you can often draw enough attention and do enough damage to let your team cap quickly afterwards.
Hope this helps an already excellent guide!
