1) Yes. It will depend upon the exact yellow filter used, but my filter (I forget which number it is) requires adding 2/3rd of a stop.
2) Two ways of looking at this. There is the technical argument, which is that after placing the lens more than it's focal length away from the film plane - the position at which the aperture value gives the correct irradiance on the film plane - bellows extension begins to have an effect. Now for most scenarios, the ideal correction to the exposure is so small, it's within the latitude of even slide film, and hence no one cares. However, the question is *when* does bellow extension need to be considered, or rather, at what extension. The inverse square law states that doubling the distance of the lens from the film plane equates to a drop in intensity to 25% of the original intensity, and hence a 2 stop increase in exposure is required (assuming this does not fall into the region where reciprocity is also required). For colour negative films like portra 160, 1 stop over or under exposure will likely not affect the colours of the reproduction by any significant degree, and will still give a fine result overall. From vague memory, and I think due to the symmetry of the problem, a drop of intensity requiring a 1 stop increase correction equates to the lens being the square root of 2 times the focal length away from the film plane (about 1.4(14) * 135mm, or 191mm). Do check this though, as it's early and my brain is not functioning on all coffee fuelled cylinders right now!
For slide film, I'd start adding correction at around +1/3rd to +1/2 of a stop required correction, as this will render in the slide, especially if the dynamic range of the scene is pushing towards the dynamic range of the film. In other words, not adding correction would potentially yield dense shadows. Take care however, as blown highlights are considerably harder to deal with than darker shadows in my experience.
