Students often feel nervous about entering a lecture theatre and there is a natural tendency to sit at the back. Sitting at the back gives you the reassuring feeling that the lecturer won’t notice you because you’re as far away as possible. Sorry to burst the bubble here but every lecture theatre I’ve lectured in has given me a terrific view of all of the students :-).
Sit wherever you feel most comfortable (or least uncomfortable!) but make sure that you can see any screens where information may be displayed and ensure that you can hear the lecturer.