People should be logging in to the vendor log, out of respect for their security system at all stores.
It depends on what I'm doing. I get what you are saying. But, if I'm doing sone mission on an app for Customer Impact, I won't unless I'm supposed to interact with store personnel. If I have to spend extra time waiting at customer service to log into the vendor book when I am only getting paid a flat rate fee, no way!! I always do sign in when making store visits as an hourly employee for merchandising companies, though.
Sometimes when I go to the neighboring states I have a difficult time logging into their vendor book. Here, I have them trained. I know where it is in most stores and I just go to it and sign in or they see me coming and they get it out for me. Except for one store, I don't even sign in anymore because the CSR at the front-end makes me wait in line with the customers. I just go to the jewelry counter there and leave my business card, the date and time. She's cray-cray if she thinks I'm going to wait in line with the customers.
I have done many of these Field Associate audits, but never at base price. I wait until they go up to at least $15. For most app jobs I sign the log, but since the WM ones in my area do not require more than 3 pics, and they are for customer service, wearing a vendor badge seems to defeat the purpose.
Incidentally, the WM and Target stores in my area do not ask for a company badge or ID, and only WM requires I wear one of their vendor badges.