Sentry bullets literally can't miss. And uh, easily defensible positions are hard to learn? Good thing they're marked by silly little non-vital things like intelligence briefcases, giant arrows pointing to such, or giant glowing capture ponts.
Or not.
Please insert quarter and try again.
Placement is important even in those locations.
(P.S. Bongo- welcome to my world.)
Not 100% sure if you agreeing or disagreeing with me. But hey.
To clarify: the most common locations are rarely marked as such, though they are obvious enough once you play a map for a little while. There's a hell of a difference between planting a sentry in the intel room and planting in a place where it's USEFUL in the intel room. Furthermore, most of the 'good' positions aren't sitting below signs of any kind, let alone ones marked "PLACE SENTRY HERE".
A good engineer will continually search for new and HILARIOUS sentry locations. Yes, the 'old reliable' positions can be good in a clutch, but lots of folks are used to those locations and expect sentries there. You also need to know how to set up a good crossfire or cover fire location in the same area if the 'n00b spot' is taken. This is all still fairly 'basic', though you can debate that last point is a 2nd level skill.
For more advanced gameplay, please see advanced ideas like: Max's "the sentry is only an alarm" playstyle, advanced forward trolling, when to tank and when not to tank, reading a map to build in an area that's NOT familiar, teleporter placement tricks, sentry range/arc tricks, environment cover tricks and terrain abuse, awareness of your team's momentum (i.e. balancing placement for maximum control of the real-estate vs. allowing yourself build time), knowing which buildings are worth your time investment at what points in the match, heroic defense of a sentry farm
under construction/playing 'combat engineer', etc. etc.
And if you sit behind a sentry for any length of time, you damn well know they can miss.