Wait till its in flower around May - the flowers, once pollinated and fertilised, will turn into the berries, so knowing where the flowers are and avoiding them when you prune will mean you get berries later. The only point to pruning this shrub is to restrict its size or keep it tidy/trained, especially if its used as hedging - its not an essential routine otherwise.

If its going to flower you should be able to see small flower buds any time now. Mine is in bud - just!.