Okay, bloodline levels are a bit complicated but here's the basics. at the beginning of the bloodlines chapter there's a table. That table shows when you have to take levels for each strength of bloodline. For instance, minor bloodlines have 1 at 1-6th level and 1 at 7-20th level (I think. I don't have the pdf in front of me right now). Now, you don't have to take any bloodline levels but once you get to 7th level, if you haven't taken a bloodline level, you get an xp penalty and you don't get anymore bonuses from your bloodline past level 6 until you take a bloodline level. So a good strategy is to take bloodline levels as late as possible in order to get the benefits from each tier without losing a level until the end. Does that make sense? I might need to grab my laptop and check the rules again to explain it better.
I have...plans for the 9-tailed jinchuuriki (cue evil laughter). Plus it's he only one that gets full bonuses from the various abilities if I remember correctly, giving the 9-tailed jinchuuriki a bit of an edge over other jinchuuriki. Mostly the first reason, though.