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 $9.30   6.348%   $9.8904   $9.89   $9.90   $9.90  
  Purchase #2 $193.33   8.216%   $209.214   $209.21   $209.20   $209.20  
  Purchase #3 $20.89   13.237%   $23.6552   $23.66   $23.65   $23.70  
  Purchase #4 $149.90   5.751%   $158.5207   $158.52   $158.50   $158.50  
  Purchase #5 $119.71   9.864%   $131.5182   $131.52   $131.50   $131.50  
  Purchase #6 $21.31   4.805%   $22.3339   $22.33   $22.35   $22.30  
  Purchase #7 $132.43   11.698%   $147.9217   $147.92   $147.90   $147.90  
  7 purchase totals = $646.87       $703.0541   $703.05   $703.00   $703.00