I am all for accuracy but I would suggest on the average light aicraft attitude indicator that you would be pushed to get a better accuracy of plus/minus one degree on a desired bank angle - and we have not mentioned turning and acceleration errors yet! (Please no theory on this one though!).
Different of course for some EFIS displays and aircraft which use inertial systems to drive attitude informtion!
Many moons ago when I did my basic training at the College of Air Training one of the first things we were supposed to do on full panel was to "calibrate" the turn needle - ie do an accurate rate one turn to check the turn needle was giving the correct indication. If not make an adjustment etc so that when you did limited panel you knew you were getting an accurate rate one turn! Not sure how this would work out on the turn coordinator which is slightly different in principle of operation.