I personally find online visits/purchases afterwards very poor for black tie events (compared to sport events) therefore always do onsite sales/printing. Even when we do small award ceremonies when photographer is on their own we will print onsite. We offer to hold prints for collection at end of evening but prefer they take them back to the table as this encourages others to come up.
I prefer to have a mobile studio, but if there is a good background in the right place then we will use that instead of or as well as. We like to shoot tethered and then direct them straight to the sales station to maximise sales. We also have viewing stations for people to view candid shots and to politely

move customers who can't make up their minds so they don't create a bottleneck.