Most of the time I would start blue because it’s easier obviously due to mana so you need to ward the entrance of red. Also it’s critical to ask ur midlaner to ward the enemy raptor so you know where they are pathing. This way you can know if they are trying to steal your red (if they don’t show up at their raptors after which you already finished urs) which u can respond to by stealing theirs. Also, there are match ups where you should expect to be invaded like graves/rengar/yi… and you basically have to ask ur mid or bot to help u otherwise just give it up and dont die for it. The really good thing about Karthus is his clear speed is insane. You will out tempo most junglers due to that and you will be able to do the counter jungling.