Yes. I'm no expert, but in my experience showers are not a drop by thing. If I had a partial time conflict, I would probably give the host a heads up for their planning purposes.
If the shower starts at 1 then you show up at 1. Unless it clearly says it an Open House than the party/shower is from the time listed until the end time listed. Typically, the hosts will plan for guests to arrive within 15-20 mins of the start time and then get the party really rolling but hopefully have appts or something ready right at the start time as everyone comes in and socializes.
Re: Bridal shower ?
~Benjamin Franklin
DS dx with celiac disease 5/28/10