Not always off.... my 19 year old Honda Pacific Coast is dead-on, per GPS. I consider it a gift, as I do know that "many" are off, but not ALL..
In the same vein as what your talking about, I did not like the fatty Ninja like back tire my 08 had. It was sort of squared off by the previous owner, and until you were rolling along at a brisk pace, the bike felt tippy as it was on and off the "square" spots of that steam roller wide back tire.
In reading and conversing with other riders, I think Alaska Jeff was the fellow I got some tire sizes from, at least for my front tire (thanks Jeff).
I too, switched to a 150/70/17 for the back. It's a touch taller and narrower. It's the same size that the Yamaha Super Tenere runs and I believe the BMW GS Adventure.
I went on the wild side for the front
, running an omnidirectional Shinko.
Both are mounted up and look smart on the rims, but the bike not totally back together as part of a rebuild so I can't yet comment on ride or speedo accuracy. I also switched to a 16T so if I was momentarily in theory dead on from the back tire switch, it's now boogered up again from the 16T swap.
I watch the speed on my GPS anyway..