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 $226.95   12.494%   $255.3051   $255.31   $255.30   $255.30  
  Purchase #2 $216.87   8.692%   $235.7203   $235.72   $235.70   $235.70  
  Purchase #3 $117.27   4.179%   $122.1707   $122.17   $122.15   $122.20  
  Purchase #4 $57.64   11.161%   $64.0732   $64.07   $64.05   $64.10  
  Purchase #5 $123.56   13.846%   $140.6681   $140.67   $140.65   $140.70  
  Purchase #6 $151.87   9.629%   $166.4936   $166.49   $166.50   $166.50  
  Purchase #7 $9.67   4.831%   $10.1372   $10.14   $10.15   $10.10  
  7 purchase totals = $903.83       $994.5682   $994.57   $994.50   $994.60