Ivan's answer would be the correct way to approach this particular issue.

However, I recently needed to implement the docIf RML flowable (in OpenERP 7.0) to guarantee that invoices generated would always be an even number of pages so they could be easily printed double-sided.  I figured I'd share the implementation, as there is very little explanation elsewhere on how to do this.

Here's the RML of the bottom of the invoice:

    </pto>
    <docIf cond="doc.page%2==1">
        <pageBreak/>
    </docIf>
    </story>
</document>

To make OpenERP properly parse the docIf flowable, you need to edit the following file:

./odoo/openerp/report/render/rml2pdf/trml2pdf.py

Before line 888 (the else block reporting that the flowable is not implemented), add the following lines:

 elif node.tag == 'docIf':
    return platypus.flowables.DocIf(node.get('cond'), self.render(node))

For reference, the surrounding code in trml2pdf.py should now look something like this (new lines in bold):

elif node.tag == 'frameEnd':
    return EndFrameFlowable()
elif node.tag == 'hr':
    width_hr=node.get('width') or '100%'
    color_hr=node.get('color')  or 'black'
    thickness_hr=node.get('thickness') or 1
    lineCap_hr=node.get('lineCap') or 'round'
    return platypus.flowables.HRFlowable(width=width_hr,color=color.get(color_hr),thickness=float(thickness_hr),lineCap=str(lineCap_hr))
elif node.tag == 'docIf':
    return platypus.flowables.DocIf(node.get('cond'), self.render(node))
else:
    sys.stderr.write('Warning: flowable not yet implemented: %s !\n' % (node.tag,))
    return None

The end result is that you can now utilize RML docIf statements in templates, and in my example, all invoices printed will always be an even number of pages.