I had the same question about pruning and was told to prune them after the new growth started. Depending on how scraggly they are, you may want to cut most of the old stuff off (some of my old stuff had sparse evergreen leaves and some of it was just ragged). I cut mine back last week.

I don't know much more about them but they're charmers and I love them. Bloomed all summer. I might have to add more :)

I forgot -- what kind of parahebe do you have? I have a couple of catarractae varieties but there's also Parahebe perfoliata, which I only know about through reading. If you have the latter, here's a couple of links:

http://www.rainyside.com/features/plant_gallery/perennials/Parahebe_perfoliata.html

Here is a link that might be useful: P. perfoliata