Ramp Room
This is a major choke point - you've got people coming from the front entrance, the water way, the wind tunnel and the sides if they've rocket/pipe jumped in. Your sentry is probably not going to survive for very long in this position, but it will do a LOT of damage. Keep close by your gun (not too close!), preferable where you'll see an assault start - and don't forget to det your dispenser if they get past your sentry.
Flag Room
Very tricky placement here, as any attacker will get a clear view of the flag "holder" before getting too close. Place the dispenser as a decoy (which can be detted if required) with the Sentry Gun facing towards the flag itself and off to one side. If you float around the entrance to the flag room (where the two corridors intersect) you can probably bait some attackers into the path of your gun.
Waterway
If you have a problem with attackers coming in via the waterway, place your Sentry Gun in this position. It will lock down just about every water attack - unless they send a sniper down to take your gun out. The dispenser will give you a few extra seconds, as no one will be able to get past - use those extra seconds to restock and warn your team, then get that sentry rebuilt.
Entrance to Base
Once again, hide the gun in a position where the average attacker won't see it - shooting them in the back as they run past. The dispenser is a last resort, as before. Be careful of RJers coming down to take your gun out, or nail grenades being thrown in.
