decltype is significantly more versatile that auto and can always be used in place of it. Therefore I think it's pretty safe to say that decltype should only be used in cases where it's completely necessary, so if auto produces the wrong result you should use decltype. Also you can't as of yet use auto in return types and parameters, so you can use decltype there as well. C++14 will significantly increase the potential uses of auto and I would guess c++17 will go further. So the situations to use decltype will only be when you need to change the resulting type of expr

Another thing to consider is that decltype isn't really necessary unless you're writing library code, auto is nice for everyday programming if you want to make your code more concise, it's up for debate wether using as much auto as possible is good, but it's virtually necessary when working with unutterable types like lambdas.