It can cause gaps/whitespace. I'm not sure of the exact browsers, but I believe netscape navigator and some versions of IE. Anyone fancy testing this?
You don't need to avoid line wrap altogether, just next to the <td> tags. And with php you can put any amount of white space anywhere:
PHP:
echo "<td>";
echo "content";
echo "</td>";
and
HTML:
<td><?php
echo "content";
?></td>
both still give: <td>content</td>