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 $213.49   13.455%   $242.2151   $242.22   $242.20   $242.20  
  Purchase #2 $61.46   10.551%   $67.9446   $67.94   $67.95   $67.90  
  Purchase #3 $107.89   3.266%   $111.4137   $111.41   $111.40   $111.40  
  Purchase #4 $73.49   3.309%   $75.9218   $75.92   $75.90   $75.90  
  Purchase #5 $109.70   10.723%   $121.4631   $121.46   $121.45   $121.50  
  Purchase #6 $111.26   6.225%   $118.1859   $118.19   $118.20   $118.20  
  Purchase #7 $246.59   4.443%   $257.546   $257.55   $257.55   $257.50  
  7 purchase totals = $923.88       $994.6902   $994.69   $994.65   $994.60