i think any art is going to compliment your training...even if they're similar...say tkd and some styles of karate...it's still gonna get you thinking about things through a new perspective...if you're training in olympic style taekwondo...you definetly need some sort of art that emphasizes holds/locks/throws...hapkido, aikido, judo, jujitsu...anything can work as long as you can find a good instructor. I must say that a lot of tkd people like to say, "we do grappling/locks/throws/whatever at my school"...usually this means they do a couple of moves every once and a great while, but they don't do them even close to enough to be able to use them without having to think about application, body positioning and so forth...just like striking, these things need to be second nature to be effectively used...before you start attacking me...i don't know what your school does...and if it does incorporate these elements on a regular basis...great, consider yourself lucky. but if it doesn't...and you don't do any cross training...don't try to use them in a real situation...cause you'll end up getting hurt trying to remember "how do i enter into this throw" or "which hand do i grab with for that lock"...if this is your situation...stick with kicking and punching...it'll serve you best