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 $67.96   12.545%   $76.4856   $76.49   $76.50   $76.50  
  Purchase #2 $58.79   4.655%   $61.5267   $61.53   $61.55   $61.50  
  Purchase #3 $89.49   9.485%   $97.9781   $97.98   $98.00   $98.00  
  Purchase #4 $98.82   5.399%   $104.1553   $104.16   $104.15   $104.20  
  Purchase #5 $78.56   3.114%   $81.0064   $81.01   $81.00   $81.00  
  Purchase #6 $136.34   3.261%   $140.786   $140.79   $140.80   $140.80  
  Purchase #7 $120.32   4.407%   $125.6225   $125.62   $125.60   $125.60  
  7 purchase totals = $650.28       $687.5606   $687.58   $687.60   $687.60