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 $122.70   5.313%   $129.2191   $129.22   $129.20   $129.20  
  Purchase #2 $79.73   10.541%   $88.1343   $88.13   $88.15   $88.10  
  Purchase #3 $97.50   9.454%   $106.7177   $106.72   $106.70   $106.70  
  Purchase #4 $224.51   4.509%   $234.6332   $234.63   $234.65   $234.60  
  Purchase #5 $9.79   3.618%   $10.1442   $10.14   $10.15   $10.10  
  Purchase #6 $227.96   7.720%   $245.5585   $245.56   $245.55   $245.60  
  Purchase #7 $143.49   11.794%   $160.4132   $160.41   $160.40   $160.40  
  7 purchase totals = $905.68       $974.8202   $974.81   $974.80   $974.70