I'd run armor pen marks and AS quints. If you want a slightly slower jungle, you can run 6 armor pen marks as well. Nocturne should not have any AD in his rune page. His scalings are actually rather poor and Duskbringer gives you a ton of AD for clearing purposes. Attack speed is much more efficient when it comes to speed. Armor pen will give you more damage against champions.
EDIT: AS also allows you to proc Umbra Blades more frequently. In most camps, you'll be able to proc it twice.

It's a free mordekaiser ghost basically. It can block skillshots, tank damage and deals quite a bit on its own. I don't think you'll ever convince me that one more point in Q is more useful than your first point in R.

I suppose. There needs to be another health item for casters imo. Rylai's is pretty terrible on her and RoA takes so long to build and stack. Maybe a late RoA? And I thought you were the one who said that mp5 wasn't necessary on morg. o.O

Viktor already has a limited inventory. I doubt I'd sacrifice that much damage for utility masteries.

Rupture scales up 55 base damage per level. Feral Scream scales 50 damage per level. There isn't much difference. The only downside to maxing W would be if you either didn't use it enough (due to range, but if you can hit Q, you can hit W after) or it ran you oom too quickly. Considering that it's a more reliable damage source and has a scaling silence duration, I'd say it's more important to level it first. Keep in mind that AP scaling really has nothing to do with which skill you max unless it lowers the cooldown, which it doesn't do for either spell. To put this in perspective, maxing W will cause you to lose 25 damage if you hit your opponent with both spells, but the silence will last a second longer and you'll be able to harass and zone much more easily.

Nocturne: More AD -> Stronger Umbra Blades in my opinion. Keep in mind that this is not my standard build, I only go this way if possible and/or if necessary. What runes would you suggest on an AD nocturne build?

Yorick: I'd replace Frozen Heart by Banshee's Veil then. Why would I take my ultimate anytime sooner? This way benefits your damage way and way more; I want to kill them without dying not die first and then go for revenge. Additionally, upgrading it has little to no use in my opinion.

Morgana: I did go with HP/lvl but this suits her way more. The MP5/lvl are quite necessary on her also the 175 hp at level 18 isn't going to cut it.

Viktor: Not my standard build, but a build I really like. It allows you to spam constantly and that combined with spell vamp allows you to stay in lane forever. You should try it in order to discover its potential.

Cho'Gath: Since his last buff, I don't feel he needs MS Quints anymore, although still a great choice. I would only run magic resist in specific matchups not all of them. I max Rupture on Cho for it's damage; even though it is unreliable. The silence is indeed huge but rank 1 or 2 is enough against most APs and if you land Rupture afterwards it is all fine. Other nice things about his Q:

• Excellent farming tool.
• Low mana cost
• Superb range
• Lower cool down.

Maxing W turns you into a CC machine instead of the damage monster you are supposed to be.

AD quints are kinda bad on Nocturne. If you're not getting armor pen, get AS. I'd take Lethality over Havoc for sure. You get IE and PD ffs.

Your yorick build has a ton of armor and no MR to speak of. I'd drop the atma's and get some MR. FoN, BV or Maw would be fine. GA is an option as well. And get your bloody ultimate earlier. I'd max it asap as well. I know a lot of people think Yorick's ult sucks, but the level-up benefits are actually kinda important.

Mp5/lvl on Morg? I thought you ran HP/lvl. It would make sense considering your low amount of health in the build and Morg's high amount of natural sustain with just a couple rings.

Dem Viktor masteries O.O

You max Q on Cho? I didn't know anyone still did that. Usually people will sacrifice a bit of Q damage for the reliability and silence duration of Feral Scream. If you're playing AP mid, that silence is pretty huge. I'd also run MR glyphs and MS quints on him, but that might be personal preference.

Uhm, I think that's all my comments for now.