We are cool with you using pennies to pay for things. As much maligned as they can be sometimes, they are still useful.
But please, use them in moderation. Try to limit your usage to four pennies per transaction. The more you use past four, the more annoyed we will be. If you choose to use pennies to conduct the entire transaction, we will become quite cross. At the very least, it will eat up vast amounts of our time and energy to ensure you have the correct amount of pennies. We may in fact elect to refuse service, and as you'll see, may go further than that. Technically, they are legal tender, but private businesses and, it seems, local governments are free to specify how they wish to be paid (though it's preferred that we specify prior to transaction). It's their call if they want to deal with you or not. If you want the pennies to be taken, you had better be nice enough about it to be deemed worth dealing with.
There was an experiment run here by a group of people in New York, to see what happened when they tried to pay a bill of about $1, give or take, with pennies. What they found in their self-admittedly-small-scale experiment was that it wasn't so much the pennies themselves. It was more the fact that it would take more work and more time to count them all. The cashiers tended not to think all that much of it until the people behind the experimenters in line started getting agitated and impatient, at which point the cashiers started feeling pressure to just get the transaction done and over with.
That was just for a dollar. Two people in the news got lucky with their pennies when paying much more. One man, Thomas Daigle of Milford, Massachusetts, was dealing with the Milford Federal Savings and Loan Association, and he was nice enough to call in ahead of time and let them know; it was something he'd been working towards for 35 years and he'd always wanted to make his final mortgage payment in pennies. They approved it.
Let me reiterate: the paying-off of a mortgage with 62,000 pennies is something that had to be specifically approved.
There's also Faith Hammock of Indiana, who has been gathering 500,000 pennies to help pay for her daughter's college tuition. The story here on that doesn't get into how she intends to turn in the pennies, but she'll find someone to take them.
But these people are the lucky ones. On the other end of the spectrum, you have people like Ron Spears of Cle Elum, Washington, who attempted to pay a $330 property tax bill in pennies and was refused. He ultimately paid in the manner of a normal person.
And then there's Jason West of Vernal, Utah, who paid a $25 medical bill- one he was disputing- in pennies. Jason was not being a pleasant person to deal with, especially since he just up and dumped them on the counter, causing pennies to fly everywhere.
West, who was paying a $25 bill, was subsequently cited for disorderly conduct, which carries a $140 fine.
And then there's the case of Frank Gilberti of Nutley, New Jersey. In 2008, Gilberti had a $56 traffic fine to pay (he wasn't wearing his seatbelt). He went out and got some rolls of pennies. The Bloomfield municipal court told him no. Gilberti persisted, pestering people to take his rolls. At one point, he was told to write his drivers license number on each roll. He asked if he'd have to do that if he paid in bills too. And then came the part where he found out the municipal court had actually put out a warrant for his arrest. And then they revoked the $90 bail after his uncle- doubling as his lawyer- said Gilberti would be pleading not guilty. (The media seems to have gotten bored with the case before finding out how it ended.)
Keep in mind, this only applies in the United States. In the United Kingdom, the penny equivalent, the 1-penny and 2-pence pieces, actually cease to be legal tender in amounts over 20 pence, meaning someone could shoot you down and have full legal backing to do so.
To reiterate: pay in dollars, or with a debit or credit card, and you won't go to jail. Pay in pennies, and we cannot make any guarantees.