Obviously taking the temperature of anything down to -50°F and up to operating temperature multiple times is going to wear it out faster. Some avionics specify not only operating temperature but the temperature range the unit can be exposed to period. In this case it is usually because of worries about high temperatures though.
If you are pre heating the aircraft before starting the units that will reduce the wear and tear. Check your avionics manuals, if they do not say anything about non-operating temperatures I would say you are good to go.