Many E6 emulsions have a yellow filter layer and/or an anti halation layer, which is composed of colloidal silver. If you just fix these films, you will get dense brown slides. If you want clear slides, you need to BLIX/STAB, or run the whole prebleach/bleach/fix/STAB program.
About embedded color couplers: yes, they are clear unless they react with oxidized color coupler. In the long run, though, they may cause trouble, so they need Formalin treatment as some point in the process chain. This is done by typical prebleach bathes in kits with separate bleach&fixer, and by STAB bath in typical BLIX kits.