Shutter speed when recording with DSLR I know that the shutter speed should be approximatively as twice as much the fps. However, I know people who tune the shutter speed to any value they need it to achieve the correct exposure. They say that, except for a few cases, using a shutter speed bigger than the recommended value (2X the fps) is not noticeable.  

I would like to hear your opinion about this. It is quite important to me, because if this is true, I would avoid using ND filters, as they reduce image quality.
On DSLRs, a fast shutter can actually be better.  DSLRs scan the image to sample rather than capturing the whole frame at once.  This is why you get a distortion when you do a rapid pan.  A faster shutter will result in stopping motion more clearly (less motion blur), which can seem a little harsher than the softness of film, but a sharper image may be preferable.  That said, a good ND shouldn't have a significant impact on image quality.

So, my final advice is, if you want crisp frames with little motion blur, use high shutter speed, if you need smooth motion for things moving around a static scene, a high quality ND is probably better.  If you simply have a fairly static shot, then shutter is probably the way to go again.