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 $80.88   9.537%   $88.5935   $88.59   $88.60   $88.60  
  Purchase #2 $117.51   13.117%   $132.9238   $132.92   $132.90   $132.90  
  Purchase #3 $122.60   9.396%   $134.1195   $134.12   $134.10   $134.10  
  Purchase #4 $65.65   3.401%   $67.8828   $67.88   $67.90   $67.90  
  Purchase #5 $169.59   12.256%   $190.375   $190.38   $190.40   $190.40  
  Purchase #6 $145.99   5.645%   $154.2311   $154.23   $154.25   $154.20  
  Purchase #7 $46.70   4.214%   $48.6679   $48.67   $48.65   $48.70  
  7 purchase totals = $748.92       $816.7936   $816.79   $816.80   $816.80