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 $225.27   10.195%   $248.2363   $248.24   $248.25   $248.20  
  Purchase #2 $115.26   12.403%   $129.5557   $129.56   $129.55   $129.60  
  Purchase #3 $146.37   9.328%   $160.0234   $160.02   $160.00   $160.00  
  Purchase #4 $86.26   3.586%   $89.3533   $89.35   $89.35   $89.40  
  Purchase #5 $47.34   13.389%   $53.6784   $53.68   $53.70   $53.70  
  Purchase #6 $44.29   4.353%   $46.2179   $46.22   $46.20   $46.20  
  Purchase #7 $238.74   4.564%   $249.6361   $249.64   $249.65   $249.60  
  7 purchase totals = $903.53       $976.7011   $976.71   $976.70   $976.70