Ideal distance to watch 3D ?

From what I know it's fine. CNET has a whole article on viewing distance so I'd check that out.

In fact the opposite is true. The closer you move to the screen, the distance that pop-out effects appear to come out of the screen will be reduced. The 3D effect as a whole will diminish as you get closer. On the other hand, the more the image fills your field of view, the more immersive it will be. It's a trade off, but my personal preference is to sit very close: 6 feet away from a 90" screen. I don't think there is a definitive ideal distance.

Here's a link that explains it better than I can: http://bsandrew.blogspot.jp/2012/09/sitting-too-close-to-your-3d-tv-will.html

i do not know about safety but few meters away from the screen would make it nicer and better for the viewers

I sit about 2 meters from my 61" set for the best results. It is really up to you but for me I like getting a little closer than normal when 3D is active.