I have already been doing this (Getting Better for each milestone). :)
But I only level up a PC if they satisfy the old school requirements of killing and looting during the milestone. My 2 PCs have reached 2 out of 8 milestones, but one of my PCs only has been levelled up once because they fled a fight.
However, they are already fairly strong now--they have been lucky and found 3 scrolls in the early going. It's possible PCs may get too strong too quickly in longer campaigns...