Here is the complete signature of grid method from source.... there is really no truncate param.
def grid(query, fields=None, field_id=None, left=None, headers={}, columns=None, orderby=None, searchable=True, sortable=True, paginate=20, deletable=True, editable=True, details=True, selectable=None, create=True, csv=True, links=None, upload = '<default>', args=[], user_signature = True, maxtextlengths={}, maxtextlength=20, onvalidation=None, oncreate=None, onupdate=None, ondelete=None, sorter_icons=('[^]','[v]'), ui = 'web2py', showbuttontext=True, _class="web2py_grid", formname='web2py_grid', ):