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 $76.59   8.863%   $83.3782   $83.38   $83.40   $83.40  
  Purchase #2 $106.54   6.858%   $113.8465   $113.85   $113.85   $113.80  
  Purchase #3 $16.46   9.777%   $18.0693   $18.07   $18.05   $18.10  
  Purchase #4 $162.99   13.717%   $185.3473   $185.35   $185.35   $185.30  
  Purchase #5 $11.52   8.405%   $12.4883   $12.49   $12.50   $12.50  
  Purchase #6 $56.66   3.518%   $58.6533   $58.65   $58.65   $58.70  
  Purchase #7 $93.50   13.529%   $106.1496   $106.15   $106.15   $106.10  
  7 purchase totals = $524.26       $577.9325   $577.94   $577.95   $577.90