being friendly and smiling and commenting with a simple hello is good enough...maybe commenting about the surroundings or whatever is not a bad thing to do because if there is an attractive girl around she sees that your like that with everybody and your not some thirsty pathetic loser...so its easier to enter a conversation with them and progress from there
but i see what your saying about flirting with every woman but i'd tweak it and tell you to be friendly to everyone when your out and about ...because ultimately you still want to be genuine and telling an old lady she is hot or something is going to make her feel like your making fun of her...if she's wearing a nice hat or something yeah compliment her make her feel good about herself as long as its genuine