I have a little doubt that maybe some of you can answer in a fly: does the "Drawing Method" (Guessing, multi-pass linear...) affect to the final rendered fractal (I mean rendered to disk) or is it for previewing purposes only? Thank you

It only affects the preview image and only during the preliminary process itself. It has no effect on the final condition of the image. That is set by your preferences before rendering (either in 'render to disk', or 'saving image as..').

The drawing method does affect the final image. For rendering use "multi-pass" or "one-pass". "Guessing" does as the name suggests (it guesses the color of some pixels), and it can result in less accurate images and nasty artifacts when it guesses incorrectly.

Especially if you are rendering to disk and using anti-alising. I had some images really lose huge amounts of detail, before I started using multi-pass. I use it for all my images now. It does take a little longer to render, but it is definitely worth it. I got the advice from Jos Leys, who knows a little bit about Ultra Fractal... ~;o}