Paying using only pennies might seem like a strange thing o do, although it is probably more common that you might think. Many people tend to think because of all the disruption and delays it would cause that paying for things using pennies might be illegal. In actual fact however the laws regarding accepable forms of payment are very different from this, and are in favour of the individual rather than the person or business being paid.
There is no reason by law that you shouldn't pay for anything with just pennies if you feel the inclination or perhaps don't like the person you are purchasing from. Some stores might even try to stop you from paying like this, but legally they cant, and could be in a lot of trouble if you decided to file a law suit against them for that purpose. Anything that has the federal mark has to be accepted as legal tender, similarly in Britain technically it would be legal to use bars of gold or postage stamps as they both have the royal seal, although i have never seen anyone using them.
The only reason something is looked down upon by society in general and whatever stores you might do it in, is because it holds people up and makes them queue for longer. When there is usually really no need, as you can always cash in your pennies at the bank or a change counter machine and redeem them for larger denominations of money. These of course are easier to pay with and don't hold anyone up, as well as being easie to carry around with you.
Some stores are actually happy when you pay with small change (not necessarily as small as pennies) because it means that they have some to give to customers who pay with larger notes. Most larger stores in fact get a delivery of small change every morning so that they have enough to last them the whole day. If they are particularly busy on a particular day however, then they might run out of change to give to customers, and in this case would be happy to recieve hundreds fo pennies.
Paying with pennies as a form of protest has also been prevalent in many countries, particularly when someone feels that they have been unfairly charged or billed. There was a famous example of a man in the United kingdom who owed a credit card company several thousand pounds due to accumulated debts whcih he did not know about. To pay them he had thousands of pennies delivered to his house, and then hired a dump truck in order to drop them all off at the bank. Because of the policies of the particular company, they had to pay the postage on whatever means of payment he used, and had to pay hundreds for the heavy load of pennies to be picked up by them.
A lot of people don't see the need to keep pennies in the general circulation or even keeping them as legal tender. What they fail to realize is that people need them just as much as they ever have. What are you supposed to do if you need change from a purchase and the amount you need back is down to a single cent, just go without? The British monetary system for example, which has a similar value to the dollar in terms of what it can buy there, has been around for hundreds of years with pennies. And they are still just as important today as they were when 4 pennies (a Groat) was the going rate for a knights daily pay.
Another reason that pennies will stick around is because of the advertising industry. Pricing things at for example $9.99 instead of $10.00 makes it look cheaper and subconsciously more people will by it. This works for all kinds of items, and makes the sales industries a lot of their revenue, people believing they have a bargain when in fact all they have is one cent less. And of course for this marketing to work there needs to be cents in order to give the change after a purchase. Getting rid of cents would in fact mean that changing the entire monetary system would be needed.
People don't tend to like paying with only pennies because it makes them look as though they are poor and don't have much money, as traditionally the largest denominations of currency are always used first, and the smallest last, when you have nothing else left to use. But then at the end of the day pennies are legal tender just like anything else in terms of coinage and notes or bills, so there is no reason why you shouldn't pay using them if you feel like it, and have a lot of time on your hands while the assistant counts them out at the checkout.
So if you don't like someone who is serving you, or you have a lot of pennies and free time on your hands, you can certainly pay with them if you want to. You may not get a warm reception or a word of encouragement from other shoppers waiting behind you, but paying with pennies is completely legal.