before actually starting it is always preferable. However, if you've ever found yourself stuck for background ideas after you've completed the foreground subject/subjects this is a great way to try different options without ruining your painting. Here I've printed out two copies of Duck Duck Goose on cheap printing paper and penciled in a couple possible backgrounds. I can even add color with Prismacolor pencils to see which colors will work best. I'm still undecided but I can continue to experiment indefinitely or until my printer ink runs out.
Another thing to consider is the intention of the artist which will impact the type of background chosen. Do I want to paint a farm scene with a few ducks and a goose in the foreground or do I want the birds to star?