I don't think that the Readers have an expectation that you will do milestones. But when you do, they will see that you are somewhat engaged with your fans. It's always great to make a shout out and thank your subscribers. 
I'm not an expert, I don't really have many subs of my comic. But it's growing slowly. When I hit a milestone, I will definitely thank them and start to give them a gift. Like a Lucky Drawing. Randomly take 1-2 readers and draw something that they want/like.