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 $57.46   13.479%   $65.205   $65.21   $65.20   $65.20  
  Purchase #2 $9.82   6.136%   $10.4226   $10.42   $10.40   $10.40  
  Purchase #3 $32.66   8.359%   $35.39   $35.39   $35.40   $35.40  
  Purchase #4 $134.47   12.205%   $150.8821   $150.88   $150.90   $150.90  
  Purchase #5 $57.66   8.781%   $62.7231   $62.72   $62.70   $62.70  
  Purchase #6 $246.42   8.300%   $266.8729   $266.87   $266.85   $266.90  
  Purchase #7 $23.90   8.143%   $25.8462   $25.85   $25.85   $25.80  
  7 purchase totals = $562.39       $617.3419   $617.34   $617.30   $617.30