Its up to you,
IsPostBack property indicates that whether this is the first time user has requested for the page or it is reloaded based on any response on postback.
This property checks for __VIEWSTATE or __EVENTTARGET parameter in Request object. if these parameters are absent that means it is requested for the first time and if these parameters are present then this request is not first request.
Code inside the ispostback will execute the first time,if you are clicking any other control of the page like button click then code inside the ispostback section would not called.