Look at DIN16901. This is the condensed experience of generation of german engineers. The default tolerances table shows how much the part dimensions differ one from another during a mold "session". By session I mean one production period - 1000s or 100s of injected part or few hours(injected in the same nest in a multi-nest mold). Let take an simple example: You have to produce on plastic case with two holes on the bottom separated by a distance of 100mm. DIN16901 says that, if the hole making elements in the tool are at 100mm, the holes in the part will be at 100±0,51mm (i am ignoring the shrinkage). This means that in the tool we do not need restrictions like 100±0,005, 100±0,02 is enough. You have to care about this, designing your products, containing plastic parts.