What kind of wallflower are they, Wilky? There are shrubby kinds, and herbaceous annuals or biennials.

pinching out the tips at any time will encourage bushy plants.

Pinching too often on wallflowers may keep them from blooming, though. The reason I asked, is that the shrubby kinds don't often need pinching, while the annuals and biennials should be pinched once when they are very young, to encourage multiple first flower spikes, later.

If we are talking of the biennial wallflowers that are sold in bunches of woody plants in the autumn for flowering the following spring, the commercial growers more often than not have already done some stopping to make them bushy so I doubt you would need to do that yourself. If you are growing them from seed, however, then they will need stopping (pinching tops out) in the summer, after they have been thinned out but before being planted out in their flowering positions.