Just got a 2015 with the 5.3L. Bought it with 170,000 miles. 90% of those miles were open highway travel. Had a new torque converter installed at the 85,000 mile mark. It runs fine. Shifts fine but should I get the tranny flushed as a preventative maintenance thing or wait till it starts to slip? I do have a powertrain warranty that covers it for next 24,000 miles/24 months.

 

Don't flush. Drain it, change the filter, clean the pan , and refill it.
Keep the old fluid just in case it gets worse, since you have high mileage.
ATF should be changed roughly every 60k