No matter what combustibles I use (sawdust, pine needles, straw, leaves, etc) I always use a (at least) double layer of newspaper as a fairly smooth place for the pots. More newsprint on top. The bottom layers are for cushioning and prolonged combustion--the newsprint because I've found it to give the blackest blacks on unglazed clay. As John said, if you know your clay, glazes, and combustibles you can pretty much repeat the same results--at will. I have one particular glaze that gives sort of a white with blue overtones--blah. However, overfire it and you get a mauve tweed that looks good with basic black. Start with a small selection of glazes and see what happens at different temps and times. For heaven's sake, have some fun!
No worries about the fun part. I will indeed have some fun. If I didn't think I was going to, I wouldn't have taken the time to get all the kiln parts and put it together.
Can you get different results with the underglaze? I was the under impression that any exposed clay, or clay like material, like underglaze is stained black during the reduction.