Always using async/await is recommended to avoid _surprising_ behavior. If a method with a signature

Task<Bar> Foo();

and it is not declared with async and it throws an exception, the exception is propagated directly to the call site. Think of this usage:

var getBarTask = Foo();

// do some other stuff

try { var bar = await getBarTask; }

catch (Exception ex) { handle exceptions }

Then if the Foo is not async the exception is thrown at 'var getBarTask = Foo();'. If it is declared with async the exception is wrapped inside of the Task object and thrown at the 'var bar = await getBarTask;'

Yes there is obviously a small performance cost. My guideline would be "always use async/await unless you call the method hunders or more times a second and the small performace cost becomes neglible. And always measure before you optimise.

edit: formating