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 $122.67   13.700%   $139.4758   $139.48   $139.50   $139.50  
  Purchase #2 $66.65   10.225%   $73.465   $73.47   $73.45   $73.50  
  Purchase #3 $185.77   4.445%   $194.0275   $194.03   $194.05   $194.00  
  Purchase #4 $10.32   8.251%   $11.1715   $11.17   $11.15   $11.20  
  Purchase #5 $83.54   11.518%   $93.1621   $93.16   $93.15   $93.20  
  Purchase #6 $234.35   8.627%   $254.5674   $254.57   $254.55   $254.60  
  Purchase #7 $191.46   3.874%   $198.8772   $198.88   $198.90   $198.90  
  7 purchase totals = $894.76       $964.7465   $964.76   $964.75   $964.90