Optional growth scan...should I ?

Luna • Baby Elle arrived 9/16/15 😍
I mentioned to my Dr I'm feeling scared about having a giant baby. She said when she feels my belly it seems like it will be a 7 1/2 - 8 Lb baby... Which is totally fine with me, I get scared thinking about over 9lbs.
She offered to give me an ultrasound at 38 wks (I'm 37 now) but warned they are often off and scare people. Should I bother psyching myself or is it better to know and mentally prepare?