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 $236.73   12.768%   $266.9557   $266.96   $266.95   $267.00  
  Purchase #2 $152.96   8.625%   $166.1528   $166.15   $166.15   $166.20  
  Purchase #3 $11.76   6.776%   $12.5569   $12.56   $12.55   $12.60  
  Purchase #4 $154.45   12.544%   $173.8242   $173.82   $173.80   $173.80  
  Purchase #5 $12.95   12.363%   $14.551   $14.55   $14.55   $14.60  
  Purchase #6 $202.37   13.495%   $229.6798   $229.68   $229.70   $229.70  
  Purchase #7 $117.83   13.571%   $133.8207   $133.82   $133.80   $133.80  
  7 purchase totals = $889.05       $997.5411   $997.54   $997.50   $997.70