[technothrasher]

For setting the a thermostat, it's not that hard to simply switch your brain over and know what's comfortable inherently in either F or C without doing a conversion back and forth. For accurate conversion, if c=(f-32)/1.8 or the converse is really that troublesome, Google can help you out.

[saagarjha]

For setting a thermostat, it rarely matters enough for you to know the exact value you want. I can get within a degree or so just by memorizing a handful of conversions and then interpolating between them.