I would recommend using CSS for this. You should create a CSS rule to enforce the centering, for example:
.ui-helper-center {
text-align: center;
}
And then add the ui-helper-center
class to the table cells for which you wish to control the alignment:
<td class="ui-helper-center">Content</td>
EDIT: Since this answer was accepted, I felt obligated to edit out the parts that caused a flame-war in the comments, and to not promote poor and outdated practices.
See Gabe’s answer for how to include the CSS rule into your page.