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 $235.98   5.560%   $249.1005   $249.10   $249.10   $249.10  
  Purchase #2 $58.94   9.681%   $64.646   $64.65   $64.65   $64.60  
  Purchase #3 $111.63   12.169%   $125.2143   $125.21   $125.20   $125.20  
  Purchase #4 $143.27   12.416%   $161.0584   $161.06   $161.05   $161.10  
  Purchase #5 $155.97   8.711%   $169.5565   $169.56   $169.55   $169.60  
  Purchase #6 $243.70   11.581%   $271.9229   $271.92   $271.90   $271.90  
  Purchase #7 $60.97   8.659%   $66.2494   $66.25   $66.25   $66.20  
  7 purchase totals = $1,010.46       $1,107.7480   $1,107.75   $1,107.70   $1,107.70