Hey there. The reason why I don't take Randuin's Omen was because it had a chance to slow attack speed down again of enemies, which I am against. I do agree about the Ninja Tabi to some extent, I do however like any extra armour early game, and that's when I pick my boots up. However, Merc Treads are normally more viable.
Thanks for your rating and comment, I appreciate it.

I'm not an avid Rammus player, but I like where this build is going. I am however, a little confused as to your choice of ninja tabi, though shying away from the Nimbleness mastery. I understand your reluctance to take dodge chance due to thornmail+defense curl+taunt combo, but if you don't want dodge, take ninja tabi out of the core build. The armor it gives is negligible, considering you have 372 armor by endgame. Either switch your main boots to merc treads or get the Nimbleness mastery. The small movespeed boost after dodging can save your life, or even net you kills if you draw minion aggro.

Rammus is not the most amazing farmer in the game. If you lane against aggressive opponents, it can be difficult to secure last-hits. I approve of your choice to start with a philosopher's stone, but I also highly recommend building a Heart of Gold early in your build (maybe after boots). Randuin's Omen is a very good tank item and is especially beneficial to rammus, considering he is an armor tank. If you're worried about the recent patch concerning gold/10 items, I believe that a Heart of Gold and a Philosopher's Stone do stack, but two philosopher's stones would not.

It's good that you focus on defense in your masteries, but your points in the other two trees aren't doing much as it is (except improved exhaust, keep that). I would suggest taking off the 3 points from Archmage's Savvy and moving them to the utility tree. Also, perseverance is a terrible mastery, you should prioritize good hands instead. Even with a heavy regen item like Warmog's Armor, you still only get a grand total of 2.48 hp/5 ENDGAME from perseverance (not including warmog stacks). That's just pitiful. Good hands on the other hand, will knock off 6-7 seconds off of every death by endgame, which is much more respectable. Spend any extra points you have in Awareness, as the extra experience is helpful on any champion.

Anyway, despite all my criticism, your guide looks pretty good so far. Keep improving it and adding more sections, like situational items. Not every game will you face a team of balanced AD and AP champions, so it would be helpful to list possible MR substitutes like Force of Nature and Banshee's Veil, including what order to purchase them and which items you should replace. Good luck on your guide; I hope to see it rise through the ranks on MobaFire. Hope some of my advice is helpful to you.

(Don't forget Heart of Gold --> Randuin's Omen, it's an amazing item!)