I tend to fall more along with Swedish thinking, that once a child is atleast 4/40 and mature enough to sit in a booster that it's safer than a harnessed seat due to the potential head loads. That being said, I have a RSTV we're going to use for travel in a few days. Ds1 is 5, 41 or 42ish lbs. We're renting, so it'll be a new car with lap/shoulder belt and he'll be in the middle. Should I use the upper tether or not? He'll be between a rfing SS1 and rfing CA.