Maybe there is way to do this and I just haven't found it yet.
I would like to be able to set the precision of my dimensions dependent on the scale of the plan, section, or detail I am in. Floor plans at 1/8" or 1/4" scale could be rounded off to the nearest 1/2", sections that are between 1/2" & 1" scale round to 1/4", and details that are 1-1/2" or 3" scale round to the nearest 1/8". The only way I've found so far is to set my roundoff for the entire project to 1/2" and then go into my sections and details and manually change the dimension text to reflect the higher precision. Autocad did this with separate dimension styles but I think it would be simple for Archicad to base it on the scale of the drawing you're in and take care of the roundoff automatically.