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 $59.63   10.621%   $65.9633   $65.96   $65.95   $66.00  
  Purchase #2 $51.79   9.660%   $56.7929   $56.79   $56.80   $56.80  
  Purchase #3 $240.77   3.651%   $249.5605   $249.56   $249.55   $249.60  
  Purchase #4 $19.25   8.411%   $20.8691   $20.87   $20.85   $20.90  
  Purchase #5 $73.30   9.660%   $80.3808   $80.38   $80.40   $80.40  
  Purchase #6 $38.60   3.735%   $40.0417   $40.04   $40.05   $40.00  
  Purchase #7 $27.97   3.727%   $29.0124   $29.01   $29.00   $29.00  
  7 purchase totals = $511.31       $542.6207   $542.61   $542.60   $542.70