In my Japanese class we were taught that one does not need to thank a cashier when they check you out or a server when they bring you your meal, but I always feel awkward remaining silent. Was my sensei wrong? Specifically:
The thing to keep in mind is that this isn't a ritualized situation, such as the 只今{ただいま}/お帰{かえ}り, "I'm back" / "welcome back", call and response pattern. When you come and go from the office or home, there are set patterns. This restaurant situation isn't like that.
Specific to your questions, there is no usual exchange between customer and cashier when paying for an item, or when a waiter brings you your food. Most Japanese I observe seem almost as if they don't acknowledge the existence of the staff and say nothing. However, some kind of nod or word would be perfectly acceptable if you wanted to do it.
On the way out, there is nothing at all wrong with thanking wait staff at restaurants. The worst that can happen is that they think you're a little unnecessarily polite, and that ain't a bad thing, is it? The words just mean what the words mean, and you can use them as you see fit, in order to represent yourself as you would prefer. Let's look at the implications:
As pointed out, you do not have to say ありがとう as you are a customer paying for a service. But you could, and it won't even raise eyebrows. You could say どうも. It's not inappropriate, but it is casual. Imagine, in English, at a restaurant saying, "hey, thanks man!" to the waiter as you leave. Does it fit the environment? You make the call.
Another one not mentioned so far that you'll often hear at casual restaurants, particularly at 回転寿司{かいてんずし}, rotating sushi places, is お愛想{あいそう} or お愛想{あいそ} which are both ways of saying "bill please".
美味{おい}しかった, "that was delicious", isn't a statement of gratitude for the service, it's a comment on the food. Say it if you actually think the food was good. What restaurant doesn't want to hear that the food was delicious?
Lastly, you can just take care of proceedings by asking for the bill by calling out お会計下{かいけいくだ}さい, "bill please", or お勘定下{かんじょうくだ}さい, "check please", or variants (you could drop the お and/or the ください) , without saying any kind of gratitude statement. Just keep it about the business, and no one will mind.
My bonus cultural observation: The Japanese concept of service is that it's not about the people. The person working at a store or restaurant becomes entirely a representative of that place while on duty, and they check their individual personality at the door. So for the customer, the staff does not merit personal interaction the way other people do.
To a non-Japanese mind, it seems a little harsh, as in other cultures, like mine, we tend to think about the person doing the job. I tend to sympathize with the guy doing the minimum wage job. However, in Japanese culture, it's not an attempt to be superior to the service staff, it's an acknowledgement that the service staff are also not necessarily personally invested in the job.
In some ways, there is a certain liberation in the concept, because the staff can also detach themselves from the job so as not to take issues personally.
Hope that helps.
Japanese here. I find it fine to say ありがとう for the first two, although どうも is more common. Not saying anything is perfectly acceptable. You can also nod, which is very common.
When you leave the restaurant, it is common to say ごちそうさまで～す or ごちそうさまでした. If you are female, ごちそうさまでした would be more common. It is perfectly ok to leave without saying anything. Nodding is very common as well.
It is true that in Kansai area (particularly in Osaka) and also in rural areas people tend to say something, compared to Tokyo for example.
I don't think it is necessary to thank them. I do though out of the habit from america. However, I have heard stories that in Osaka you do thank the waitstaff and cashiers.
I've seen Gochisou used more as an indicator to the staff that you are done with your meal and ready to pay, though that is just an observation. I haven't heard Oshikatta or arigatou used as often though. Though this may be a difference if you are eating at the counter or at a sushi bar.
For first two, you can say ども, but not ありがとう because you don't have to thanks them, since you are using their service, and you are customer.
And you can use ごちそうさまでした / 美味しかったです at restaurant if you wish to, and may be just whisper those if you don't want other people looking at you for some reasons.
I always start with お早うございます、こんにちは or お晩です (Tôhoku-dialect), and finish with あざ〜います。 With some kombini clerks, I add some comments on the weather. In return, they are nice(r) to me, and every one is happy.
Well, I'd often ask for her age/number/mail… More seriously, I just say something like "はい、ど〜も〜". But I tend to enjoy interacting with the staff. Last time, I ordered "your favourite". And when he came with the order, I said "I hope you have good taste, or I'll complain!" and everyone laughed. I consider those jobs as very tiring, so I try to make it agreable for them too. (oh, I should mention that letting them choose the order is not being very nice here; rather an embarassing situation I reckon.)
I always yell my "ごちそうさまでした". I wouldn't say 美味しかったです unless I had something incredibly delicious (so, I never say it). I wouldn't thank them either, since I already did the gochisou part. I expect them to thank me in return.
