Underground tank is filled to 88%, is it OK?

Living in west maryland, has a 500 gallon underground tank, weather forecast says big winter storm is coming this weekend, so requested a propane delivery today as my UG tank is lower than 20%. The driver came around 4:30p which is a little bit dark already, after he finished, we saw the gauge shows the tank is at 88%, since we normally only filled up to 80%, so we asked him is it OK to fill to 88%, he says with weather this cold ( the next 7 days the temperature will be no higher than -1 F with big snow on Sunday and Monday), it's OK to fill up to 90%. So just want to ask here is he telling the truth? are we in any safety risks? We normally used 4 gallons per day, so it would take us around 4 days to consume 3% and bring the gauge down to 85%. thanks