It is a very difficult question to use AC adapter or not

It is a very difficult question to address, as it depends either way. You can do either or, but if you always use AC then it doesn't really matter. So long as you charged it from when it was dead then there should be no problems. Most laptop batteries such as Apple Laptop Battery, Asus Laptop Battery or Dell Laptop Battery have a weird charge function where they somehow mess up charge estimates when you haven't drained your battery, so if that's the case you're battery life will be shorter, but it doesn't make it unusable. Additionally, if the battery is full, then the AC should bypass the battery charge completely. So, if you're using AC all the time it shouldn't really matter, depending on how much you want to use it later away from the AC may make a difference, but not significantly.