hubbahubba, that depends on the environment, if there is sufficient lighting and CO2 in the water they can be cut back at any stage. The two reasons why plants grow fast when they hit the surface is more light and mostly, easier to access CO2. High lighting and high CO2 levels can give you similar results under water, but there is always a little advantage in getting CO2 from the air Vs water.
With this plant, I usually try to cut the longerst tips off early so that the lower stem starts to branch out, in the meanwhile other stems will cover the branch until it has bushed out. This is one of the best candidates IMO for cutting the tips off and leaving the base, it really bushes out a lot, if you can cut it in phases or replant the tips so to cover the base until it has filled in you will get the nicest growth IMO.
Hope that helps
Giancarlo Podio