I don't feed, they normally have suffient forgage worker/brood ratio to feed them selves adequately and normally have at least one super of open/capped honey
If no stores, then yes, feed!
Thin syrup, and don't let the feeder go dry while they are in comb-drawing mode.
Once they stop fast comb-drawing, check that they have some stores before withdrawing feed.
What you don't want is for them to fill plural frames with syrup (or on the other hand for them to risk starvation, or even limit their comb-drawing for lack of fuel.)
/// ADDED - "thin syrup" - about 45% sugar by weight.