When I worked at a racing/high perf engine building shop we occasionally built an engine that made more power than the sum of it's components would suggest it would be capable of. When I crewed on an SCCA Pro Sports 2000 racing team those cars were all powered by a stock 2.0L Ford engine. Depending on whose sealed stock engine you bought (no changing their internal parts per the class rules) they made 140-145 HP, which is at least 50% more than those same engines made in the early Pintos.
The difference in those engines over their peers is that they were blue-printed. They were 'trued'. Minor deviations in their tolerances were corrected or "moved" to the most beneficial part of their tolerance range. Those engines that were not blueprinted still made a lot of power, but the difference in otherwise identical builds was noticeable both on the dyno and in driving them.
A trued action is no different. Some are closer to the ideal set of dimensions and will shoot well with no truing. Others, while still within production spec, are far enough away from the ideal set of dimensions that they will shoot only fair. Measuring them to understand where they are within the acceptable range is almost as much work as just getting on with truing them. So if you have an action trued by someone who knows what they're doing you'll have a known foundation to build on. If you do not have it trued then you won't really know how good or bad it is until you shoot it.