Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $21.33   3.159%   $22.0038   $22.00   $22.00   $22.00  
  Purchase #2 $242.30   4.737%   $253.7778   $253.78   $253.80   $253.80  
  Purchase #3 $9.45   12.706%   $10.6507   $10.65   $10.65   $10.70  
  Purchase #4 $244.87   7.523%   $263.2916   $263.29   $263.30   $263.30  
  Purchase #5 $17.87   12.857%   $20.1675   $20.17   $20.15   $20.20  
  Purchase #6 $176.47   13.509%   $200.3093   $200.31   $200.30   $200.30  
  Purchase #7 $195.25   12.852%   $220.3435   $220.34   $220.35   $220.30  
  7 purchase totals = $907.54       $990.5442   $990.54   $990.55   $990.60