I reward all recalls. I always have. It may save their life.
It doesn`t have to be a treat - it can be praise or a play. Why not graduate the rewards?

I also reward all recalls. If the dogs are close with little in the way of distraction they may get praise/stroke. Calling off a chase or away from dogs/play, I throw a party!

I haven't been able to much recall training with Cain, but I did a lot with Moon. I may be wrong but I don't think SB means you need to switch to a new treat (eg cheese to sausage) but you need to take a few different treats with you at the same time. Our dog trainer always advises to have at least two different treats on you so that if the dog gets bored of the first one, you can pull out something better. I tend to take a mixed treat bag eg liver cake, hot dog and cheese, so the dog never knows what they'll get as a reward, it keeps them interested. With Moon I'd give chicken for an easy/normal recall but I also have pepperoni (very high value!) for super brilliant recalls eg instant fast return, or called away from chasing birds etc. I also used to use jackpot recalls from time to time eg 10 pieces of pepperoni, for something utterly amazing (like recalling from a squirrel). Always worked well for us.