That article I just posted can explain it better than I can, me, I just take my darks as a matter of course, some I have used for a while, others I take when I am imaging.
As I said, just make sure they are the same as your lights and you will have no trouble, and no, no need to be so fussy

you will be mapping any thermal noise anyway.
Most of all, enjoy taking your shots.
Cheers
JG