Hi Chris
If you have a thermometer on your imaging rig (even just a cheap one attached with velcro) then no reason you can't assemble a darks library - for Canon DSLRs you're probably ok if within 2-3 degrees, or even 5 deg.
Remember you can shoot them on cloudy nights, or inside in a closest (or even the fridge).
My wife was puzzled one night why the air con was going full bore when everyone was asleep upstairs. Said I needed more darks at 19 degrees and seemed a fair enough way to get them
