Is symmetrical current perfectly symmetrical beyond analysis?
In general you'd have to do a sensitivity analysis, and different circuits might well have different sensitivities. One way is to start with a theoretically perfect circuit and perturb each of the component values slightly and look at the resulting change in whatever parameter you are considering. That can be done with SPICE, for example. 

While some simple circuits might have relatively low sensitivity- you change the resistor value by 0.1% and the current mismatch in two branches changes by about 0.1%- it's possible for the sensitivity to be much, much worse, even in linear circuits, for example, if two almost-equal voltages are just about canceling, a small change in one of the voltages can result in a large increase in current.