I mix it directly into the fix bath. Why? Because first off, selenium toners (which already include Thio) will bleach and tone horribly at full strength, but drawn down to 1-2% (which seems like nothing, I know) and mixing it in...never had any issues and you can visibly start seeing changes after 4 minutes or so.
Another good trick is to use oxalic acid...very low concentration...umm..have to check some data sheets...but a few drops of that into the VD solution during the coating and BAM, increase contrast, speed, and a bit more DMAX.