Depends if you're looking short term or long term.
I would venture that the Tories will be privately happy with a few token Ed Miliband victories along the way as it increases the chances of him still being Labour leader cometh the general election in 2015.
The worst case scenario for Cameron is for Miliband to do badly in these mid-term elections thus paving the way for a triumphal return to the helm of David Miliband.
That said, I'm hoping that Miliband wins the next election to pave the way for Boris in 2020! :-)