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 $246.83   7.172%   $264.5326   $264.53   $264.55   $264.50  
  Purchase #2 $46.52   10.285%   $51.3046   $51.30   $51.30   $51.30  
  Purchase #3 $229.50   12.349%   $257.841   $257.84   $257.85   $257.80  
  Purchase #4 $20.88   13.697%   $23.7399   $23.74   $23.75   $23.70  
  Purchase #5 $111.94   5.568%   $118.1728   $118.17   $118.15   $118.20  
  Purchase #6 $60.75   13.614%   $69.0205   $69.02   $69.00   $69.00  
  Purchase #7 $63.42   5.458%   $66.8815   $66.88   $66.90   $66.90  
  7 purchase totals = $779.84       $851.4929   $851.48   $851.50   $851.40