Maybe. There may be issues with heat flow leading perhaps to localized elevated temperatures. Also, you've got a bunch of components on that circuit board. Some of them are light, some heavy. Placing the unit vertically may place addtional stress on the heavier components that are attached to the circuit board which can result in who knows what. Time to get more creative.
Most receivers rely on natural convection too keep everything within safe operating temperatures. The heatsink fins would be aligned vertically and the receiver's top cover would be vented to allow unrestriced airflow. If you place the receiver on it's side, you'll alter it's ability to properly dissipate heat.