Remember the question is for taking off into IMC, so in a way contacting radar immediately after t/o could be ok (As our TWR control is limited in IMC anyway). However, I hate it when pilots are already off my frequency when I call to change them. Even in good weather, if Im launching 2 planes close, I want them both talking to me, not to someone else!!
I guess what our own experiences are will bias our decisions somewhat here, but I can think of a scenario where it would be important in IMC...
Think of a very slow aircraft departing.. even in the worst vis and LVPs, the inbound will be at least 2nm when the departure is overhead the Loc. Everything seems fine and slow a/c is already off frequency. At 1nm, the inbound suffers windshear and goes around. By the nature of the windshear, they cannot accept a turn and want to fly straight ahead.
I dont think I need to carry on to show the point. If that a/c cannot stabilise enough, he will go screaming straight towards the departure! Highly unlikely you might say, but thats not the same as impossible. And the same situation could be MUCH MUCH worse in good vis and non-IMC conditions!!