When I build for a regulated mod, I build on it as well. I always turn it off when mounting the coils though, I turn it back on when I want to read the ohms, and then pulse it.
When I build for my mech, however, I build on top of my Coil Master ohm reader. I always consider a .02ohm margin, but that's just me being paranoid, it's probably more accurate than that, but I don't really wanna risk it.