In your opinion is it considered rude to not smile and simply say "how can I help you?" While working behind the counter

According to my boss that's rude and I should be cheery and welcoming to customers. I am welcoming I'm saying hi what can I get for you. I just don't smile or don't find it necessary to be animated. On another note if I'm busy making drinks I'm not going to look up to greet everyone. I just want to focus on making good drinks ok don't bug me.