White balance is very important. Often essential to get right in-camera.
You get a feel for what the colour temperature should be so you can set your cameras white balance accordingly. Eg: You're indoors using Blondie 2K's and Redheads, you set to 3200k. You're using HMI's exclusively, 5600k. You're outdoors using only natural light with no cloud coverage during the middle of the day, 5600k, etc. Of course there's also a creative choice in what you set your camera to. You don't have to be locked in to the 'correct' setting, although it's a good place to start.
Also depending on your cameras codec and colour bit depth you have more flexibility in post for correcting white and black levels. If you're shooting 8bit H.264 you better make sure you get your white balance mostly correct in-camera, but if you're shooting RAW you can almost afford to white balance entirely in post (not good practice though).
Also 18% grey cards are mostly for exposure, not white balance. Use a piece of pure white card for a reference point in post. You can get greycards that have pure white, pure black and 18% grey on them. That's a good choice.