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 $190.76   11.776%   $213.2239   $213.22   $213.20   $213.20  
  Purchase #2 $66.96   3.165%   $69.0793   $69.08   $69.10   $69.10  
  Purchase #3 $38.99   12.134%   $43.721   $43.72   $43.70   $43.70  
  Purchase #4 $34.88   4.704%   $36.5208   $36.52   $36.50   $36.50  
  Purchase #5 $126.85   5.559%   $133.9016   $133.90   $133.90   $133.90  
  Purchase #6 $232.30   9.660%   $254.7402   $254.74   $254.75   $254.70  
  Purchase #7 $68.91   11.589%   $76.896   $76.90   $76.90   $76.90  
  7 purchase totals = $759.65       $828.0828   $828.08   $828.05   $828.00