My wife is of the opinion that sweet peas should have the tops pinched out as they are planted on to make them busier. I say she shpuld wait until they have reached the height you want them.
As soon as the plant has about 4 side shoots you can start pinching the tops.It encourages the plant to throw out more side shoot and then more flowers.
I grow my sweet peas from seed and always pinch out the top when the plants have reached about 4" high. This makes them throw out side shoots which are the best ones for flowering.