The Millers, who made it four wins from five pre-season games with a 2-1 win over Bolton Wanderers on Tuesday, open their league campaign away to Crewe Alexandra on August 3.
"I don't believe in friendlies and I think you should play every match to win but I thought there were a few outstanding performances," Miller told the club's official website.
"It is just another step on the road to being ready for August 3. You have to have a mix of opponents, that's why we went to Scotland last week and playing Championship sides will offer us something different."
Rotherham finished second in League Two last season.