I've completely forgot to tip many times while visiting the US. It's just not something that I ever do in my country. And then you learn it's not only restauran

I've completely forgot to tip many times while visiting the US. It's just not something that I ever do in my country. And then you learn it's not only restaurants, but bars and, for some reason, the person behind the cashier at a Wallgreen's is sometimes also tipped?