I'll rephrase for clarification: You'd better start reducing power and pitching forward 1000' early so the aircraft will not exceed 250 knots (that's the speed limit below 10,000') or you will not blow past your assigned altitude by more than 200'. ATC doesn't like that. In the 200, even if you're light you can wait a little longer.
You can wait to reduce power and start your pitch forward, but it's going to be a fun ride and your captain might get very excited. I wouldn't know by experience or anything.