When I first started container gardening, I tended to overpot. I wanted to make sure that the plants had plenty of growing room
I lost a lot of plants that way, so I started filling in around the main plant. I love a contrasting ground cover at the base of a plant, for example. It also lessened the number of pots that I had to take care of and water.
The web is full of gorgeous and unexpected combination pots, as is our own NGA site
I see things that other people do, and I think, yeah, maybe I can do something similar. None of my pots are ever going to be on the cover of Southern Living or anything, but I have fun doing it.
I do have some pots with just the one plant in them. Usually it's a specimen plant that I picked up, and I think it's pretty all by itself. But for the most part, I find myself trying to match plants so I can group them in the same pot.