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 $19.56   9.542%   $21.4264   $21.43   $21.45   $21.40  
  Purchase #2 $39.94   10.505%   $44.1357   $44.14   $44.15   $44.10  
  Purchase #3 $19.84   5.672%   $20.9653   $20.97   $20.95   $21.00  
  Purchase #4 $108.57   4.362%   $113.3058   $113.31   $113.30   $113.30  
  Purchase #5 $241.77   8.277%   $261.7813   $261.78   $261.80   $261.80  
  Purchase #6 $95.82   6.213%   $101.7733   $101.77   $101.75   $101.80  
  Purchase #7 $64.75   11.557%   $72.2332   $72.23   $72.25   $72.20  
  7 purchase totals = $590.25       $635.6210   $635.63   $635.65   $635.60