You can trim them gently to shape in the spring, but risk losing the flowers - or wait until they've flowered and do it then.

I do the latter, the flowers are too lovely to forfeit by doing it early.

Yes, I agree - but if it's getting out of shape or outgrowing its space, it's a possibility.

The label on my sambucus 'black Lace' says to prune to desired size in winter. I only bought it last summer so it isn't big enough to need pruning. It also says that if there is another variety of sambucus nearby eg in a neighbouring garden, the plant will produce attractive berries for the birds. If you cut it after flowering you will lose the berries. I have now bought a sambucus 'Black Beauty' and the label on that says it can be pruned to the ground in winter if necessary, but otherwise just cut out the dead wood.

There isn't another Sambucus near me yet i get berries every year but yes, you'll lose the berries if you prune after flowering.
I know of a lot of people who prune their Sambucus to the ground each year. They make huge amounts of growth in a year .... easily 6' so pruning won't hamper it's growth, quite the reverse !
I have Black Lace which i love.

That's encoureaging Louise as I didn't really have room for 2 but as they can be pruned hard I'll see how it goes!