That is a dichotomy. Use a string trimmer to bring down the high spots. The reason for not running a mower over it is to avoid shifting the pieces around just as the new roots are extending into the old, underlying, soil/sand. Don't run out over the new sod and slide into place with your string trimmer. Walking normally on it should be fine.

If it were my lawn, I would let it all grow until it was knit to the soil underneath and take more than 1/3 off. I have been experimenting with tall St Augustine for a couple years. It seems to get up to 32 inches long/tall and then stop growing. Now I'm keep it "manicured" down to 12 inches with the string trimmer. It looks good and rarely needs water. Oh and my house is on the edge of the Texas desert, so it is an experiment in reduced water needs by allowing it to grow tall.

Thanks for the reply. I was leaning towards letting it root. I definitely want to leave it alone as much as possible now that the lawn care company applied chemicals without my authorization.

Interesting experiment. I've never heard of keeping St. Augustine that high. I was planning on keeping mine at the maximum height on my mower (4 inches) for the same reason. A lot of people in this area keep it too low and water constantly this time of year. They also tend to scalp the lawn.