Although wearing a tie makes you look the part, they very rarely get washed and are therefore a contamination hazard. They do get trapped in machinery very easily so need to be tucked away.
I prefer to wear an open shirt in the lab and don the tie for meetings or "dress" occasions, ie when the press are about.
If you are worried about patients or other staff not knowing who you are then you should show your ID badge. This is compulsory in our gaff.