Boot Stretching

My new Lange RX130s were heated with boiling water to soften them up for the stretches that I needed. The boot fitter told me that I shouldn't warm or heat them up in my Hot Gear Bag because my stretches would revert to back to the original form of the boot. Reading the suggestions on the Zipfit website where heating up your boots is recommended for fitting their liners, it's mentioned that stretches made with the boot heated to 110-120 C won't be affected by heating one's boots in a heated bag. Looked just about everywhere for some information regarding this issue, but haven't found anything specific about plastic having a memory if they were stretched after only being heated to 100 C.