A lot of people partially restrict airflow, but Suburban warns against restricting flow or covering vents.
If you don't let enough hot air out of the furnace, the over-temperature safety sensor may activate shutting off the burner. If it repeatedly activates, you may end up with a weakened sensor and an expensive service call to replace it.
Also, if you constantly create an over-temperature condition inside the furnace and the sensor should ever fail to open, you could have a very serious problem.
If your furnace only runs occasionally, maybe the risk is low. If your furnace runs a lot, the risk of having a problem might be higher.
So, if you do this, make a note to yourself in case you start having furnace problems down the road.