From Boeing FCTM;
On airplanes with FMC U10.7 and earlier, after flaps and slats retraction is complete, select VNAV or set the desired climb speed in the MCP speed window. Before selecting VNAV, flaps should be retracted because VNAV does not provide overspeed protection for the leading edge devices.
With FMC U10.8 and later, VNAV can be engaged anytime because VNAV provides overspeed protection for the leading edge devices.