Michael,
this depends on a variety of factors including;
size of block
composition of tissue
thickness of sections
The most important of these is the composition of the tissue. To minimize
chatter and compression  it is prudent to either orient the tissue
appropriately and/or leave adequate amount of wax around the specimen so that
it can be carefully trimmed.
two good general rules are
1. to cut through tissues in order of hardness with softest first. This is
not always possible if for example you are going to cut through a tooth in
its bony socket. In this case you have to make compromises.
2.    to introduce the tissue as gradually as possible to the knife edge so
that the knife does not encounters a large areas of tissue all at once.

There are exceptions -  if you are carrying out length measurements you may
want to orient the block so that the length that you are going to measure is
horizontal to the edge of the knife. Mos of the compression (usually in order
of 5-10% will be vertical and may not be possible to eliminate by expansion
of sections).

Barry

Michael Manzer wrote:

> Sheehan's "Theory and Practice of Histotechnology" has a chart on
> proper orientation. I would think the pathologist would prefer reading in
> an orderly fashion instead of zig-zagging. Good luck!