When is the preferred time to prune bearing grapefruit trees?

Answer 1

Commercially, they would be hedged any time now, after the crop is harvested and before the bloom in march, if possible. You would expect a lighter bloom and crop the year following the hedging operation.
For a dooryard tree the timing is not so critical. You will likely have some fruit on the tree most of the year, so you can decide how much fruit you want to prune off. Of course you could prune dead wood, root sprouts, etc., any time.

Answer 2

winter