Bear in mind we cruise at constant Mach no, not IAS. speed of sound (c) is directly proportional to square root of temp (in degrees kelvin). M=v/c, so TAS=Mc. so at constant M, TAS is directly proportional to square root of temp. Degrees kelvin are celsius + 273. You'd need quite a substantial change in OAT to get a big change in TAS; -60C is 213K, so a +/- 5C is only +/-2% and then there's a square root in there.
thnk have got all that right - long time since my engineering degree, or indeed CFS groundschool!!
Hope I'm adding to the sum of knowledge and not muddying the waters
DH