I think it depends more on which paper you're using and the response that paper will give you. All papers will show improved Dmax with selenium, but some will also show a color change, especially warmtone papers. For example, a dilution of 1+19 might work well with a warmtone paper for slightly improved Dmax when little color change is wanted, where as a 1+9 dilution might be better for toning a neutral paper, such as Ilford MGIV which I believe responds better to lower dilutions of selenium.
That all being said, I use one dilution, 1+9 for anywhere from 1 minute to 20 minutes depending upon the paper (mostly Ilford warmtone & MGIV), but I also like color change in my photographs. Hope this helps.