Table 6.1 allows a 5/16 fillet weld to be undersized, for a maximum of 1/8, provided the undersized portion of the weld doesn't exceed 10% of the weld length. Another consideration is web to flange welds on girders, where undersized fillet welds are prohibited at the ends, for a length equal to twice the flange width. 

Also, worth mentioning is what the AISC has to say...  "Fillet weld terminations should be detailed in a manner that does not result in a notch in the base metal transverse to applied tension loads that can occur as a result of normal fabrication. An accepted practice to avoid notches in base metal is to stop fillet welds short of the edge of the base metal by a length approximately equal to the size of the weld. In most welds, the effect of stopping short can be neglected in strength calculations. There are two common details where welds are terminated short of the end of the joint to permit relative deformation between the connected parts:

Welds on the outstanding legs of beam clip-angle connections are returned on the top of the outstanding leg and stopped no more than 4 times the weld size and not greater than half the leg width from the outer toe of the angle.

Fillet welds connecting transverse stiffeners to webs of girders that are 3/4" thick or less are stopped 4 to 6 times the web thickness from the web toe of the flange-to web fillet weld, except where the end of the stiffener is welded to the flange. Details of fillet weld terminations may be shown on shop standard details."