We encourage sitting after surgery, and patients are encouraged to sit, even on the day of surgery. This will not stress the incision, and may actually help the incision heal by opening up the gluteal fold near the anus and allow air circulation. Most patients find that sitting on something firm is more comfortable than something soft. Soft furniture (like a sofa) allows you sink down into it which presses on the incision and is often more uncomfortable.
Some patients have a tendency to sit sideways, on one buttock, with the thought that it will be better for the incision. Actually, it is worse for the incision and is not recommended.
Dr Immerman does not recommend a coccyx cushion, and would rather that you sit on regular chairs. However, the goal here is not to cause pain, and if you find that a coccyx cushion is more comfortable, it is OK to use one.