No, you just have to make sure the function you pass is associative.

Only in really strongly typed languages like Agda can you make the associativity constraint /part of the type/ of the function, and thus statically checked by the compiler; but it's perfectly reasonable to make it part of the specification of the function's behavior that it expects its argument to be associative. (Or, slightly more generally, that the order of reduction is unspecified; in which case only an associative function will produce a totally deterministic result.)