I would go 100, 250, 500, 1,000, 1,500, 2,000, 2,500, 3,000, 3,500, 4,000, 4,500, 5,000, etc.
But I never really post post milestones that much.
I'd only do 100 because it's a mini start, and then 250 is really the start itself. And so on, and so on.
And for reputation, I don't post my rep as a milestone.