Many countries have laws specifying that certain denominations are legal tender only up to a certain total amount. The US does not, but you can run into other legal trouble. Consider the contractual obligation of good faith, for example. You might potentially be liable for the additional expense incurred in accepting a truckload of pennies.
" A payment of coins is a legal tender throughout Australia if it is made in Australian coins, but this is subject to some restrictions about how much can be paid in coin. According to the Currency Act 1965 (section 16) coins are legal tender for payment of amounts which are limited as follows:
* not exceeding 20c if 1c and/or 2c coins are offered (these coins have been withdrawn from circulation, but are still legal tender);
* not exceeding $5 if any combination of 5c, 10c, 20c and 50c coins are offered; and
* not exceeding 10 times the face value of the coin if $1 or $2 coins are offered. "