I do both. I do a lot of experimenting in the material room itself, and I too tweak by looking at the preview.

But, just as an example, I experimented for hours in the material room a couple years ago making my candle shader. I got close but eventually I realized I couldn't get to the right look without the math. At that point, I went into matmatic and produced a totally believable glowing candle material in 20 minutes. As soon as I loaded it, I didn't need to tweak anything. It was right, because the math was right. This was a case where artistic subjectivity was irrelevent. I wasn't trying to produce a look that was imagined. I was trying to produce 100% realism, and that involved matching the laws of physics precisely. Guessing at the solution through experimentation wasn't getting me past 80% realism.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)