I have some parent forums with only one sticky post (then closed) and I didn't want the separator then, but I did in the sub-forums ;-)
I did a manual change to the FORUMDISPLAY template as follows...
before...
after...
Code:
$threadbits_sticky
<if condition="$totalthreads>0">
<tr><td class="thead" colspan="7"> </td></tr>
</if>
PS : I did this in 3.6.8...
PPS : Now to try the other suggestions here ;-)