I work part-time in retail, and I'm expected to acknowledge a customer within two minutes of them coming to the department. If they say their fine, thats OK, but it drives me nuts to see staff chatting when people are looking for help. Even if someone is after no more than some advice, or something small, they need help, and you never know what they might buy next time they come in (and of course they might be one of the richest women in the world!). Just saying hello is often enough.
On the other hand, British people do seem to hate asking for advice. I work in a place which is known for customer service, and yet people (OK - blokes) will wander around the place looking at all the TV's we have, from the 32in to the 55in (unless you buying five at a time, its one or the other), having said 'We're fine/ I know about these, etc' when they patently don't have a clue (they will often be a couple, with the wife having a terrible fixed smile of despair). I'm paid to advise people, and I actually like helping them, yet there customers who would rather die than talk to you, or get angry if you point out in passing even the most basic information. They they come out and moan to everyone they know all about the terrible service...
If you get bad service, complain (not winge). If you get good service, tell us, its nice to hear it.