If your house is properly insulated the others rooms will be warm just because your living room is. May as well heat them all as the cost difference is minimal and then you can be comfortable that other 10% of the time.
When I first read this something about it stuck in my brain, but I couldn't put my finger on it until just now.
>May as well heat them all as the cost difference is minimal
If you sit down and reason out the math, you find that this isn't actually true.
The total heat loss is proportional to the temperature difference integrated over the total area of the building envelope (weighted by U-value). The integrated heat flux through this surface must equal the heat supplied within the interior, primarily by the heating system.
If rooms with exterior walls are colder (when in the equilibrium maintained by the heating system), then by definition you're losing less energy through that surface, so therefore less energy must be added.
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The advice also wildly disagreed with my own experience, which is that shutting off unused rooms (especially rooms that are naturally colder anyway) has resulted in large energy savings.
I do make sure to regularly monitor such rooms for any sign of moisture or mold, but blocking off the vent/under-door airflow (and keeping a particularly chilly guest room closet ajar) seems to suffice.
