I almost never swing a fly so giving slack, even if I didn't think it generally a very bad idea, isn't an option. On the rare occasions I do get a take on a swung fly it's undoubtedly my favourite take and you get time to enjoy the whole process. I can usually tell if it's well hooked like this as well. Sadly the number of takes you get is often severely curtailed by fishing a swung fly.
By and large when retrieving the best tactic is just to carry on until it all locks up. When swinging I just clamp the line until the fish is on then lift. I would say that if you are not landing 80% of your takes you need to have a good look at your tackle and techniques. Fresh grilse can be very tricky but you should still land 3/4's. Small fine wire trebles hook and hold very much better than anything else I have used and in combination with a soft actioned rod gives you the best chance. Neither give nor take line and you should be there.
The biggest difference between a good and a very good angler will nearly always been down, in large part, to the ratio of takes to fish on the bank.