I don't see any benefit to pruning such young plants. Paniculatas are tolerant of heavy pruning since they bloom on new wood, but that doesn't mean pruning is required.

It is not necessary to prune them every year, but you will want to prune out crossing branches and those that do not add anything to good shape. I prune in early Spring just as the new growth buds are beginning to swell - after winter when I can seem them, I cut back to a pair of strong buds to shape and control the overall size.

When pruning little lime hydrangea, do you cut the stem on an angle above a bud or just straight across?

Rose pruners tend to cut at an angle (45 degrees) but with hydrangeas, I just prune so the cut does not let water accumulate on top of the pruned part.

Great, that makes perfectly good sense to me. Thanks Luis.