The win keeps North End fourth in the table and firmly in the promotion hunt while County slip to the foot of the League One table.
Grayson admitted afterwards that he had toyed with the idea of using Hume from the start before finally opting to hold his Canadian striker back as an impact sub.
"I chose to keep the continuity going knowing that players can come off the bench and affect the game.
"I'm happy that Iain Hume got his goal, it was fully deserved.
"I thought both of them changed the game. Iain Hume and Joe Garner were lively, they were positive and I gave them something to think about when they came on.
"Any game you go into, you've got to earn the right to play and be disciplined in what you do. And I thought we were professional, we played from the shape of the team.
"We threw a team out which I wasn't sure how they were going to play, given a few injuries and suspensions that we had."