My understanding of the prestige as it works currently, based on my observations on my own build, is that you never actually lose prestige anymore. Chisel shapes either give prestige or don’t, depending on how often they’ve been used in a build, see below for more about the way chisel prestige bonuses work currently:
In any case, to be brief about it, what seems to happen these days is that the same type of block does give progressively less prestige for every additional placement of that same type of block but you will always be able to continue accruing prestige regardless of how much of a block you’ve used. As suggested, variety is “important” to “maximise”.