Regarding installation temperature, can anybody explain what is the preferred practice to determine it?

The plant will be built in March when the ambient temperature is likely around 10 degree C. The piping material is A333 Gr 6 since the ambient temperature can drop down to -40 C in winter. There might be shut down for maintenance during winter time and the temperature with insulation on piping hopefully drops to -20 C. Design temp is 150 C.

Given the situation, what installation temperature should I take? -40 C is blamed by my fellows for being too much conservative. 10 C seems to be too high. Can I take -20 C as an installation temp since maintenance and start-up is another installation process (from shrinkage to expansion)?

Many thanks in advance.