Ninja is a poor subjob for Warrior before level 30 (at the earliest). At level 24 you get Utsusemi: Ichi... which mandies will rip right through (even goblin smithies will give you a hard time with their double attack). With Dual Weild I, you're attacking as slowly as a great axe and probably doing less damage. You also lose your ability to effectively hold hate, relying totally on Provoke and Utsusemi (which is a pain in the *** to recast a second time during any fight with a monk type mob, or one that double attacks... like the mandies and smithies).
If you're only using Warrior as a subjob, don't bother with Ninja: you won't need it. If you want to take Warrior past 37, then you'll want to eventually invest in Utsusemi so you can have ninja as an optional subjob as you close in on Warrior 50.
Your best bet is to sub Monk through the jungle at least. War/Pld doesn't give you anything War/Mnk doesn't give you (besides a Cure I: the first defense bonus you get from Pld doesn't stack with the one from Warrior), but War/Mnk gives you a few extra tricks War/Pld is lacking (Boost, for instance, which is like a mini provoke every 15 seconds). Honestly, War/Mnk is probably the best tank for the jungles, with Paladin coming in second.
If you don't want to level monk, then don't sweat it too much: at that level, just about any melee can act as a tank with the right gear, so you could take War/Pld through the jungle. When you hit 30 though, you'll be kicking yourself. In Garliage, you're only effective means of holding hate will be Provoke (you won't have enough MP to cure for hate... as a 55 War / 27 Pld I would have about enough MP for 2 cure II's and a Cure I ><... it wasn't until 48 War that I could even milk 2 cure II's out of a paladin sub, and I'm hume). That's just not enough when the mobs are hitting you as hard as they do in the citadel. You could sub Nin at this point, but you'd want a Sharashi belt to enhance dual weild.. which adds another 350k or so onto the pricetag of subbing ninja.
So what it comes down to is that going War/Pld would be the easiest (in terms of effort), and also least efficient way of reaching 37 on Warrior. War/Nin would be the most expensive, most time consuming (farming up 500k or so for all the equip and tools you need) way to go, and would end up being only a little more effective than War/Pld (you'd get 6 shadows to play with at the beginning of each fight... after that, it's back to you, the mob, and provoke).
If you want to play War well, and without investing a ton of gil into equipment, your best bet is to either level Monk up to 18 so you can go War/Mnk to tank straight to 37 (and trust me, past the jungle, it will get frustrating... but thats why you need all the hate keeping tools that War/Mnk provides) or level up Thief to 18 so you can go War/Thf to DD once you hit 30, and take that the rest of the way.
Another option, if you have it, would be War/Rng. This isn't a tank class: you'd be there as a DD (although Sharpshot does pull hate, it doesn't make up for losing Boost). This would be viable right up to around 40, so it could take you to the cutoff point for subjob level. Of course, you might not have much luck finding parties ; ;
For the jungle, another option would be War/Drg or War/Drk. These are again, meant for damage dealing, not tanking, although as with War/Pld, you'd be able to get by as a tank, but there are better ways of doing it. Drg and Drk both offer War an attack bonus around level 20-29 (at level 30, warrior gets an attack bonus that doesn't stack with the one from these two jobs, so at that point it's less viable to sub these jobs). Drg also offers Jump (jump will still do nice damage subbed, at least around htat level range), so it would be slightly better than Drk at this level range, as far as damage dealing goes (although the added Str from Drk is nice, as is Drain). But like War/Rng, it would be hard to find parties with these combos (most people wouldn't understand your reason for subbing them, and would assume you were teh suxor n00b.) At 30, if you wanted to stay a DD, you'd want to sub Thief for sneak attack.
But again, if you want to tank, the best advice I can give you is to level up Monk. I know it's not the answer you want, but the other alternatives aren't very good :\
