As a rule you should always empty your BC completely before you start you ascent. And during your ascent you should continue to try to release air to account for expanding air inside your bladder during ascent. Failure to do so could result in an uncontrolled and buoyant ascent.
If a diver completely empties his BCD, then there won't be
any air inside the BCD during the ascent, right?
I disagree with the empty-your-BCD-completely-before-ascending philosophy. Consider that a drysuit is another buoyancy compensation device. No one advocates squeezing out 100% of the gas inside the drysuit at depth to protect against an uncontrolled, buoyant ascent. Most drysuit divers just set their exhaust valve to automatically dump and ascend under control, dumping gas from inside the suit as necessary. No big deal. (Yeah, I realize that dumping gas from a drysuit in an emergency might actually be easier than from a BCD, i.e., pull open neck/wrist seal.) I suppose that's why I don't feel compelled to dump 100% of the gas inside my BCD prior to ascending. I begin my ascent at neutral buoyancy and simply manage the BCD bubble by dumping gas from the BCD as necessary on my way to the surface.
The only time you should be adding air to your BC is for minor buoyancy adjustments and when on the surface (such as starting and ending a dive).
Around here, divers have to wear substantial exposure protection (drysuit or thick wetsuit). Assuming that a diver is properly weighted and wearing a thick wetsuit, he'll probably find himself adding a
substantial amount of air to his BCD during the course of a dive, particularly at the beginning of a dive (full tank) and if his dive profile is rather deep (100+ fsw or whatever depth significantly compresses his wetsuit).
Dive conditions vary across the globe. Certain environmental factors, e.g., water temp, can influence gear choices and how a diver conducts his dive. This includes how much gas he decides to keep inside his BCD at any given time.