Everyone has their own rules of thumb about site prep based on their own personal experience. I've only had one infected site and that was back in 1997 not too long after I started on the pump. It was annoying enough and took long enough to get rid of that I still tend to try to be careful.
The thought of just inserting without any type of prep boggles my mind, but to each their own. You don't need to be obsessive, just wash with soap & water and possibly/probably also use alcohol or some other antiseptic.
I try to do my insertions after I've showered so that I've had a chance to soap down not just the new site but the rest of me as well. I also swab with alcohol before inserting. If it's overkill and not needed, that's no big deal to me. If I'm going to err I'd rather it be on the side of caution, at least when it comes to preventing site infections. A little extra time to clean the site seems a small price to pay to avoid the big annoyance (or worse) dealing with a site infection can entail.
I really am not sure what "little plasters" might refer to so can't say anything about that. To me, "plaster" is something you put on the walls inside a house, or possibly the ceiling.