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 $243.83   13.334%   $276.3423   $276.34   $276.35   $276.30  
  Purchase #2 $230.99   8.175%   $249.8734   $249.87   $249.85   $249.90  
  Purchase #3 $120.76   12.874%   $136.3066   $136.31   $136.30   $136.30  
  Purchase #4 $61.87   7.421%   $66.4614   $66.46   $66.45   $66.50  
  Purchase #5 $229.89   7.595%   $247.3501   $247.35   $247.35   $247.40  
  Purchase #6 $139.29   8.784%   $151.5252   $151.53   $151.55   $151.50  
  Purchase #7 $10.86   11.468%   $12.1054   $12.11   $12.10   $12.10  
  7 purchase totals = $1,037.49       $1,139.9644   $1,139.97   $1,139.95   $1,140.00