I would suggest taking 1 level of Rogue or Factotum (Dungeonscape) to give you trapfinding, and that's it.
Things to note:
Swordsage gives you Greater Invisibility already. A level 2 Shadow Hand boost maneuver called Cloak of Deception gives you Greater Invisibility until the end of your turn. Greater Invisibility doesn't go away if you attack. You can use this maneuver every battle, or even more often if you recover maneuvers. Dipping into Ninja will give you less Invisibility uses per day than just going Swordsage gives you and Sword sage gives you Greater Invisibility.
Swordsage already gives you an amped version of Ninja's Great Leap with a level 3 Tiger Claw stance called Leaping Dragon Stance. It makes you considered as always running for jumps, and gives you a 10 FOOT enhancement to your jump distances. This is important because many (or all?) of the high level Tiger Claw jumping maneuvers require you to jump as high as your foe's height. Without Leaping Dragon Stance, this is impossible or incredibly difficult, especially if your foe is Large size or bigger. Great Leap gives you a +4 bonus on jump checks. Meaning you get +4 feet with long jumps and +1/2 foot on high jumps. Great Leap is always and in every way inferior to Leaping Dragon Stance.
The loss of 2d6 Sudden Strike is not very significant, and Rogue provides the in all ways superior Sneak Attack if that is the only reason to take Ninja.
Basically, Swordsage is Ninja but better in all ways except it doesn't have trapfinding or poison use. Poison Use isn't all that great because the most cost effective poisons have a low save DC (Drow knockout poison, roach paste) and thusly are worthless at higher levels, and all the other good ones cost an incredible amount per dose (Black Lotus Extract, Wyvern Venom cost 1000's of gold). Acquiring it with a feat is much better than taking 3 levels of Ninja if you really want to focus on it with your character. If you want to focus on poisons, check out the Poison Handbook for all the information you could want:
http://brilliantgameologists.com/boards/index.php?topic=4854.0So if you want to be a skill monkey, take 1 level of Rogue or Factotum. Since you already like Able Learner, combine that with Factotum. They get "All skills" as class skills, no kidding. Say hello to Iaijutsu Focus (Oriential Adventures) and all the skill monkey skills possible!
Not being able to write "Ninja" on your character sheet does suck, but it's really for the best.
Also, if all you want is Shadow Hand maneuvers, might I suggest going Ruby Night Vindicator (Tome of Battle, prestige class) instead of going Swordsage? If all you want is Shadow Hand maneuvers, Ruby Night Vindicator is very superior to Swordsage. Using Cloistered Cleric 1/ Warblade 3/ Crusader 1 to meet prerequisites and then applying Ruby Night vindicator maneuver progression to Warblade maneuvers sounds pretty spicy (Cloistered Cleric is a Variant Character Class in Unearthed Arcana, page 50). Think of Ruby Night Vindicators as more battleready versions of Swordsages with cleric spellcasting added on top for flavoring. They get full Base Attack Bonus, d10 hitdice, access to Devoted Spirit+White Raven+Shadow Hand+Stone Dragon and can sneak around with full armor. They also get cleric spellcasting (Have I mentioned this enough times?).
Edit: Go Factotum 1/ Cloistered Cleric 1/ Warblade 2/ Crusader 1 ... to enter Ruby Night Vindicator for skill monkey ability.
