It’s usually a good idea to get people to sign in. That way if anything is missing, at least you have something to show to the police though it’ll probably be a fake name.
Most people don’t tend to put their full info down, so I don’t know how well you’ll do with getting an email address.
There’s always a nice debate on what condition the house should be in before showing it. As the saying goes, you never get a second chance to make a good first impression.
If you have a sign out front saying open house or adverstised that the house will be open for showing and have indicated that it might go for less than appraisal or FMV (i.e. not retail) then some of your viewees might be potential buyers for other houses. You need to get their info or card. You probably will need at least another person to help corral them. I missed out on some the last time I showed a house thus, my buyers list has suffered. That is potential money out of my pocket.