http://i.imgur.com/0DCQP1B.jpg
http://i.imgur.com/OYKlNwT.jpg
These are the most popular spots (snakewater arranged by how frequent I see it being used)
The sentries indicate how useful they actually are, level 3s usually end up with 1 pick where the level 1s might not even work. Just make sure you're able to swap back instead of dying with the sentry. It's going to slow them down but you shouldn't die to keep it up.