Generally speaking, avoid rows. It usually looks better to have a group (or "drift") of the same plant together. When planting multiples in the same area, odd numbers look better than even numbers.

That said, when I have a large plant like a gaura (I didn't believe the tag when it said that single little plant was going to get 24-30" wide in the course of the summer), I do use just one.

Drifts of plants that overlap side by side, longer than wide, and maybe repeating across the length of the bed would be the way I would plant it. If they look a bit more like swirls rather than blocks, it looks more natural.

Thanks all, good suggestions! I like the idea of drifts, I can see where that would be better than planting in rows. And longer than wide - does that mean the drifts would be elongated following the depth of the bed (4') or the length (25')?

BTW - here is a picture of the bed:
{{gwi:27162}}

As you stand away from the bed and the fence and look at the 25' side of the bed, the drifts would be wider from left to right than they are from front to back.

I should add to my earlier comment about odd-numbers that odd numbers looking better is more the case when you can see the individual plants; if the plants grow together in one undifferentiated mass, it doesn't matter if you have odd or even numbers.

Fantastic - thanks!