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 $56.85   6.548%   $60.5725   $60.57   $60.55   $60.60  
  Purchase #2 $47.49   10.326%   $52.3938   $52.39   $52.40   $52.40  
  Purchase #3 $147.30   11.616%   $164.4104   $164.41   $164.40   $164.40  
  Purchase #4 $86.34   7.782%   $93.059   $93.06   $93.05   $93.10  
  Purchase #5 $132.28   6.772%   $141.238   $141.24   $141.25   $141.20  
  Purchase #6 $128.78   6.330%   $136.9318   $136.93   $136.95   $136.90  
  Purchase #7 $43.52   11.205%   $48.3964   $48.40   $48.40   $48.40  
  7 purchase totals = $642.56       $697.0019   $697.00   $697.00   $697.00