is that expensive to observe it at application:didFinishLaunchingWithOptions?

[wuyongrui]

- (void)observeReachabilityStatusChange {
    Reachability *reach = [Reachability reachabilityForInternetConnection];
    reach.reachableBlock = ^(Reachability *reach) {
        NSLog(@"reachable change %@",@(reach.isReachable));
       // ...
    };
    [reach startNotifier];
}

what i want to know is the code befor in the didFinishLaunchingWithOptions cost a lot or the cost can ignore? or should i do that in the viewController where i need ?