I've only been playing a short while and I've pigeon-holed myself by only playing a small set of maps (upward, badwater, barnblitz). I don't know other maps well and I'm looking for an experienced engineer to show me around some of the other maps to give me an idea of ideal sentry spots.
I've watched highlander matches on youtube to see what other engineers do but sometimes I don't understand why they choose the positions they do. Sometimes I will blueprint their setup and people I'm playing with will complain about the location, so I'm come to the conclusion that these engineers I've watched have other reasons I don't understand.
For instance, look at the sentry placement in upward in this video at 3:50:
Why did he choose this spot? Wouldn't the spot where the heavy and Pyro are hanging out be better? Or is it better to retreat fully to the very top of the hill on the tracks (basically giving up the third)?
I will compensate you for your coaching time.
While Mr. Burger's placements can ultimately be seen as subjectively effective due to this specific game's circumstances, there are definitely situations where a more aggressive team or more coordinated flanks could've proven BLU team more effective, as is the case with the combo/sentry hold on hill, or Mr. Burger's 10hp escape. 
In regard to the third hold, it's suuuper aggro ... probably should have been shut down when Mr. Burger gets sniped, but it seemed that BLU was losing players around then. While it's generally good for sentry to cover cart, such placements like those are moreso easily spammed out and it's generally better to set up far back, where combo can control cart (demospam is really big when holding upward third), and the offending team would have to put more of a commitment to the sentry.
