I agree with most of the guys comments, especially "tye with whatever materials you like to work with". This is to say, if you are fishing with the flies you tie. If however, you are going to show, then you should tie the "Traditionals' with only traditional materials (as close as possible - some of the feathers are impossible to obtain).
Comments on a couple of I have learned in tying and fisning soft hackle flies. If you are tying black, use anything that is comfortable for you. If you are using Uni-flos, I suggest you take it off the spool and split the floss (to remove bulk) and tye with it as if you were tying hackle and keep it flat, not twisted. The floss will not have a tendancy to bunch and ride up on itself as you are laying it down beside the previous wrap and not over it. This also helps to make for a more sparse body on your fly.
Silk is not terribly expensive when you figure the yardage on the spool and the number of flies you can tie. The main advantage to silk is it does become more translucent when wet.
Someone mentioned that when a fly is wet, the bronze of the hook will darken the color. True. However, the Alec Jackson hooks (expensive but lower prices can be had by shopping) are chrome and they really make the wet fly glisten when tied with silk.
If you are really into soft hackle flies as I am, here is a web sight you might really like. It is full of good information and Mike connor will reply your questions in reasonable time.
Tight lines and hope this helps.