Hi. I didn't get involved in proposal writing until I became a faculty member. It involved a group proposal to the funding agencies, so I received some mentoring in that process. But I did not get any exposure to this as a grad student or post doc. Having some experience there would have been most helpful. The Early Career proposal effort is very challenging and competitive, so having mentoring is immensely invaluable. And DOE and NSF have distinctions, with NSF having two merit criteria (both intellectual merit and broader impacts) that must be emphasized..