I think it's important to tell the viewer what they can expect if they subscribe. I hear people say, 'like and subscribe' all the time. But I seldom hear people do a little elevator speech so the viewer knows why they should subscribe. 'If you're enjoying this video and have an interest in ____________, please give us a like and subscribe.' I don't know how well it works, but I think it's best to tell them why they should subscribe.