Looking at your terminology and subsequent responses, it seems
that your nomenclature may be confusing.
From an Engineering perspective,
"a grid pattern in Excel as precisely as possible down to about .005
inches" means that you're trying to create a series of horizontal and
verticle lines with each .005 inches apart... (which is roughly the
thickness of a hair)

A 600dpi printer will TRY to produce this, but it means that IF it
uses ONE dot for the line, ONE dot for the space, then ONE dot for
the next line, you've hit your .005 from OUTSIDE to OUTSIDE of the
lines...

Now, subsequent responses seem to indicate that you are actually
looking to make a grid in which the GRID LINES are .005 thick.
this MAY be possible, but I wonder if the printers, in their highest
quality mode, are using more than 3 pixels for a line.

I think if it's going to be at all possible, you're going to want to
use a printer that's a photo quality printer, that usually goes up to
1200dpi. Then, instead of using Excel's borders, just select to
print the grid lines (under Page Setup -> Sheet)