I mean, you “should” do what the book says, cause that’s what the game’s math assumes. So, max at 1st level, choice of rolled hit die or average-rounded-up each level thereafter. Personally, I’ve always been bothered by the fact that the rolled option is statistically worse, so I allow players who choose to roll for it to reroll 1s, which brings the average up to the same as the fixed value. That balances the options without impacting expected PC hit point values.