Chalky: That may be common (and sensible) practice, but I'm not sure if it's written down anywhere. Bearing in mind that we are not technically allowed to use radar in the hold, we are talking about a procedural service, where everything is based upon reports. So if you say you are leaving then we can give the level to somebody else. If you leave it to 2-300 feet then I suppose to report that you HAVE LEFT would be more appropriate. If it is crucial you might be asked for a 'Report passing'. I have certainly almost been caught out by a (foreign) airline saying 'Leaving' and then taking some 30-40 seconds to do so.
Don't confuse the 2-300 feet with the 400 feet we would need to see on radar (outside the hold, of course!) before we can allocate your level to someone else who is not 3 or 5 miles away laterally.
------------------
"Take-off is optional, Landing is mandatory"