If you already have 9xx series cards, by all means use them - I just wouldn't bother BUYING any of them for a mining rig unless they were SUPER SUPER cheap - and I'd be iffy about it even then.
I'd also ignore anything below the 980 completely as far as buying cheaply goes.
A < $100 960 is worth picking up for instance. They don't usually draw more than 60% TDP anyways.
Not when you factor in the cost of the system, and that a $200ish dollar 1060 is going to earn quite a bit more with less power usage on the same things a 960 can mine effectively on.
With that said, I DO have 3x 960 and a pair of 950s - but those were bought before the 10xx series had been announced, and have probably paid themselves and their share of the systems they are in off by now in the longish time I've had them.