The reason there is such a wide variance in the spec manual for the ‘as installed’ bearing to journal clearance is because of the normal tolerances associated with the journal diameters, bearing holes, and the bearing thicknesses. When you have several variances that work against you when putting them together, you have what are called ‘stackups’. A worst case scenario for the clearances being on the tight side while all the associated dimensions are within specification are a top of the spec journal diameter, bottom of the spec bearing hole, and top of the spec bearing thickness. Likewise, the opposites of all these puts the bearing clearances on the loose side.
Tighter clearances just require some extra thought in regards to the oil viscosity being used. Standard weight oils being used with tight clearances can cause an overheated bearing due to inadequate oil flow through the bearing. As Kenneth mentions, spun bearings now come into play and pretty well ruin your day. From a performance standpoint, tight clearances will not be as free reving as with looser clearances. There's some free horsepower and fuel economy here with the proper bearing clearances. I’ve actually torn a stock street engine down in the past that was not running up to expectations and simply added an additional 0.001” clearance to the mains and rod bearing clearances which made a significant difference in how the engine ran afterwards. As a general rule, better a little loose on the clearances rather than tight.
If I’m not grinding a crankshaft to a specific journal size as a result of performing a size check with the bearing halves installed in their respective holes, then I’ll grind the crankshaft journals on the low (small) side of specification. This is worth an easy 0.0004”-0.0005” in additional clearance with the crankshaft still being ground within specifications. There are those instances where I’ll grind the journals either a ½ or 0.001” under the low side depending upon the intended use or application of the engine. This usually occurs after performing a size check of the bearings in their bores and finding the clearances to be on the tight side if the crankshaft is being ground within specifications. The connecting rod bearing clearances can also be manipulated by resizing the big end rod holes either towards the large or small side of the spec rather than honing them to size using the middle of the spec value. There are a multitude of ways of altering the clearances but measuring with anything other than a good set of mics and/or a dial bore gauge will not get you there. Plastigage is not accurate enough for any kind of serious blueprinting. Just had to throw that out there.