If you have contacts that you don't require to add a signature but do want their approval you can use the 'Approve only' option.
In this case SignRequest will add this approval to the signing log, but it will be easier for the user / signer to quickly approve the document. It also has the added benefit that there will not be a visible signature on the document of that person.
You can select 'Approve only' in the advanced options or in the prepare window.
On the main window, click 'Advanced' and select 'Approve only':
Or in the 'Prepare view' click the icon: