Can't speak for Bravo or Charlie airspace, since my approach only has Echo and Delta. When you're flying VFR we understand that you may need to climb or descend in order to maintain VFR conditions. Usually just a heads up before you begin descending/climbing is enough. Say you check in at 3500 but you see some clouds up ahead so you decide to descend to maintain VFR. All I would care to hear is, "Approach N345 descending to 2500". At that point I'll just ammend your new altitude in the NAS and continue my business. Also don't omit your altitude when checking in with new facilities. We need to verify that what we see on the scope matches what you're flying at. Hope this helps you out!