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 $14.41   12.824%   $16.2579   $16.26   $16.25   $16.30  
  Purchase #2 $219.86   13.232%   $248.9519   $248.95   $248.95   $249.00  
  Purchase #3 $112.42   4.869%   $117.8937   $117.89   $117.90   $117.90  
  Purchase #4 $246.53   5.673%   $260.5156   $260.52   $260.50   $260.50  
  Purchase #5 $6.94   5.177%   $7.2993   $7.30   $7.30   $7.30  
  Purchase #6 $56.71   10.107%   $62.4417   $62.44   $62.45   $62.40  
  Purchase #7 $96.71   9.447%   $105.8462   $105.85   $105.85   $105.80  
  7 purchase totals = $753.58       $819.2063   $819.21   $819.20   $819.20