For assemblies of parts like a "stackup" where each part has a tolerance, but you rarely see all parts out + or all - , you can do a more detailed analysis, if worst-case stackup won't get you within the tolerance that you need. But for stuff I designed, I had to make sure that it worked even at worst case stackup. Geometric Dimensioning and Tolerancing (GD&T) really improved on this, because it takes all factors into account, and you make your part function drive your tolerancing, and your gaging and fixture design, so they all match up. If the "GO" gage works, the part will fit in production. A good example is "tolerance of position" in GD&T, a hole diameter versus hole location; If the hole is near the high limit in diameter, you can blow the location of it a bit more, whereas at the low limit in diameter, the location needs to be more precise, so that the bolt goes through adjacent parts. I.e., it related diameter to location, whereas the old way, you just had +/- on the diameter and +/- on the location, with no relation to each other.