As to pinching out the wallflowers, yes it does improve the plants. By
removing the apical shoot (or meristem), the plant diverts its growth
hormones (auxins) to create new shoots lower down, thus you get a
bushier plant with more flowers. It applies to more than just
wallflowers though. However, if you're growing show Dahlias or
Chrysanths, leave the apical shoot intact so that you get the one large
flower required for the show bench.

Now for once I don't agree with you Spider.
In the dim and distant past we used to grow around 5000 wallflowers to
sell for planting out in the autumn. they were treated like cabbage
plants, sown in a seed bed then pulled and planted out with a cabbage
planter so that the young plants were spaced about 9 inches in the row
and 4 rows to a bed. given this sort of space they would naturally bush
out without any stopping, I've known us having plants where we could
only get 2 into the old Orange boxes that had a wired up lid on them.
(These were plants we put into auction at the local market).
As for chrysanths and dahlias.
If you are growing these for showing you stop them once to get 3 stems
per plant then dis-bud as required. Remember that with dahlias there are
several sizes in show classes
Pom pom……………………Maximum dia. 2 inches
Miniature………………….Maximum dia. 4 inches
Small…………………………Maximum dia. 6 inches
Medium …………………….Maximum dia. 8 inches
Large………………………….Maximum dia. 10 inches
Giant………………………….Maximum dia. 12 inches

David @ a still almost dry side of Swansea Bay