Thinking about Eagle routing grids vs various component spacing.

For through-hole components, placing components on a .1" or .05" 
grid and then routing on a .025" or .0125" grid is adequate.

For PLD devices, the PQFP package has lead spacing of .65 mm
(25.6 mils) and an SMD width of approximately 15 mils.  The TQFP
pacakge has a lead spacing of .5 mm (19.7 mils), and an SMD width
of approximately 13.8 mils.

The clearance specification for dis-similar signals cannot be more 
than the lead spacing less the lead width, or the package leads 
themselves will trigger a design rule violation, even with perfect 
trace placement.  25.6-15 = 10.6 mils, and 19.7-13.8 = 5.9 mils.

Since traces always appear centered on the routing grid, half 
their width plus the inter-signal clearance plus the distance to 
the center of the lead must clear the edge of the next lead, which 
is to say, the lead spacing less one half the lead width.

The distance between the routing grid and the center of a lead 
is bounded by half the smaller.  There are cases where the skew 
is always smaller.  (For instance, if the routing grid divides 
the lead spacing, the skew will always be zero.)  I haven't been 
able to fully characterize a lower limit, though.