Most industry standard transceivers don't have an assured output level with open inputs, some like MAX485 however have so-called fail safe behavior. For all others, you should place bias resistors, otherwise the receiver will have serious difficulties. Without parallel termination, the bias resistors can be quite high, e.g. 10 k. Problem is that they won't suffice if you later decide to add parallel termination, neither the inbuilt "fail safe" termination will work in this case.