I am thinking about building another houseboat in the near future. The
pontoons on boat i just build were hexegon in shape with a tapper front
>this, the back is flat or butt or this shape [.

would a < at the back reduce drag. It was a graet deal of work
making the front like this> so i don't want to do the backs unless
it has some advantage.

Probably not as a sharp break limits suction. Full size pontoons carried to the end maximizes displacement which is IMHO more important.

It's interesting to note that there have been some pontoons built with a reverse break at the rear which looks sexy but is of dubious value unless you operate at planeing speeds where the extra surface area becomes useful.
OLD HOUSEBOATER38595.5386574074

I had a 24' pontoon boat years ago with tapers on the rear. It did seem to help at take off and at planing speed as it did not sit down in the back as much on applying hard throttle and once up to planing speed I could tell a difference compared to others with cutoffs in the rear. Not sure if this makes a big difference with a pontoon houseboat. I have never seen one with enough power to get up on plane. For that you need a Gibson or Harbormaster or similar design.