I am wondering what is the standard. I suppose that being flush (no pun intended) could be a standard, but somehow I think that there should be some sort of small but discernible distance.
I have a 50 yr old house with a newer 1.6 gallon toilet and a 12" offset. Take the lid off the tank and a paint roller goes behind the toilet quite comfortably. Saves a lot of work and mess when repainting.