Granite is cheap if you find it in the form of tossed out surface tables. I thought to myself "that will never happen". Immediately found several on CL. One guy was selling a 6'x4' Starret inspection surface, along with some smaller ones including the one I bought. Gorgeous. The only guy who had showed interest had wanted the 6x4 for his woodstove!
That said, I don't really see why one needs one. I guess one can say that about any single piece of the puzzle, one needs something, but not necessarily any one part. I design in CAD, not bikecad at this point. I also consider my fixture part of the picture, as I sorta consider space while staring at it (I don't do customer bikes, but obviously if I was doing a numbers based build I would follow those, but for looking at one-offs I hold stuff in the jig), it also holds my stuff in position for assembly. I check straightness in a variety of ways, I use stuff that fits to my welding table which is a 48 inch long milling machine table, but only 8 inches wide. I don't really find the inspection grade granite surface that useful for bikes, and that is before discussing the terrible things people do to them. I'm keeping mine along with my cast straight edges to do repairs on the surfaces of my machine tools, like I otta...