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 $241.78   11.239%   $268.9537   $268.95   $268.95   $269.00  
  Purchase #2 $117.41   4.389%   $122.5631   $122.56   $122.55   $122.60  
  Purchase #3 $26.97   3.234%   $27.8422   $27.84   $27.85   $27.80  
  Purchase #4 $146.58   10.516%   $161.9944   $161.99   $162.00   $162.00  
  Purchase #5 $7.81   5.114%   $8.2094   $8.21   $8.20   $8.20  
  Purchase #6 $119.62   4.277%   $124.7361   $124.74   $124.75   $124.70  
  Purchase #7 $101.65   5.808%   $107.5538   $107.55   $107.55   $107.60  
  7 purchase totals = $761.82       $821.8527   $821.84   $821.85   $821.90