I do it with almost ALL my beds.

Impatiens or petunias in front, smaller 2 1/2' to 3 1/2' tall dahlia and then 4-5' dahlia in rear. Then I mix in a Calla Lily or 2 or glads.

My thinking is that individual and varied colors within a bed make for a visual 'over load' of colors so I color 'coordinate' my beds. If it is a bed of purplish dahlias, then the Calla will be purple as will the petunias. If it is my 'yellow/gold' bed, then yellow marigolds or yellow snapdragons may be mixed in.

Just make sure there aren't so many that they 'steal' water from the dahlia roots and cause the dahlia stress.

I sometimes border the front edges of the Dahlia beds, but even planting 18-24" apart, once the Dahlias get going, I don't think there is enough sun for even impatiens within the bed. I have been growing bedding Dahlias from seed to use at the front edge, and have lots of petunias this year that may get placed in a few spots. Most Dahlias are such "show offs" that it isn't easy for me to mix them with my other plants.

I think it may not be exactly "under the plant" but maybe in front or next to it. The reason is that my dahlias get pretty bushy and can probably block out good light to any plants directly under it (or as the case may be, when it first starts out, it looks like it needed filler but as the season goes on, the dahlia takes up at least 1-1.5 feet width.

So yes, just plant around the dahlia to add color or complement to the dahlia area.