All X-RAY's can be blocked standing with a few exceptions:
Sheeva: be off the ground when she jumps, you should jump, fly, float, do whatever involves being in the air
Jax & Kano: These two are grabs, they can be avoided by jumping backwards, I heard they can be stopped by a sweep aswell but that takes super-human, matrix timing so just jump it.
Johnny Cage: I thought I'd put this here since it's still different to the rest, just don't hit him and you should be alright, I believe you can knock him out of it if you hit him with a projectile though. Most (average) cage players will use the X-RAY when they are being rushed down constantly so watch out for that, I've never played a good Johnny though so I don't know they'd use it.
Also, OP said about blocking low hitting X-Rays but they all hit high don't they? aside for the above, of course.