As far as showing the bona fides of the marriage, yes rental agreement will be one evidence to provide.
It is a matter of priorities if you are more concerned about maintaining the terms of a rental agreement than obtaining lawful permanent resident for your wife, then certainly make the right choice for you.
Is it not something you both have to discuss and decide what is in the best interest of the BOTH of you?
Immigration process needs to be pursued with seriousness to obtain the anticipated result and it is always best to get it right the first time.
Just food for thought.
You are advised to contact an immigration lawyer to discuss your concerns, if you have suficient evidence it might not be necessary to have the rental agreement as evidence.