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 $218.86   13.779%   $249.0167   $249.02   $249.00   $249.00  
  Purchase #2 $113.57   8.397%   $123.1065   $123.11   $123.10   $123.10  
  Purchase #3 $132.26   5.228%   $139.1746   $139.17   $139.15   $139.20  
  Purchase #4 $202.92   4.106%   $211.2519   $211.25   $211.25   $211.30  
  Purchase #5 $155.78   13.723%   $177.1577   $177.16   $177.15   $177.20  
  Purchase #6 $96.31   13.859%   $109.6576   $109.66   $109.65   $109.70  
  Purchase #7 $242.34   3.789%   $251.5223   $251.52   $251.50   $251.50  
  7 purchase totals = $1,162.04       $1,260.8873   $1,260.89   $1,260.80   $1,261.00