I prefer if it's not, actually. Not a fan of the fireball-throwing. I don't find very obvious magic to be particularly believable. It has to be something I could see happening in the real world for it to really have any effect on me. Same reason I find serial killers scary and zombies funny...
Whether posting fiction online is a good idea or not depends a lot, I think, on your work style.
For authors with a slower more methodical, one-thing-at-a-time, focused work style, hoarding first publishing rights may be very important. For me, I am well aware that I am throwing away first...