Per IRS Form 1040 Instructions, page 21:

Rounding Off to Whole Dollars

You can round off cents to whole dollars on your return and schedules. If you do round to whole dollars, you must round all amounts. To round, drop amounts under 50 cents and increase amounts from 50 to 99 cents to the next dollar. For example, $1.39 becomes $1 and $2.50 becomes $3.

If you have to add two or more amounts to figure the amount to enter on a line, include cents when adding the amounts and round off only the total.

For items listed individually (i.e. on Schedule B) and transmitted electronically individually, each amount is rounded with 50 cents or more rounding up to the next dollar and 49 cents or less rounding down. In some situations this can cause a $1 rounding difference.